Last Updated: 12/3/2010

Through December 31 2010, Bad Weasel Games is donating $1 to The Water Project for each copy of Snow Day Sudoku sold!  The Water Project is a US-based non-profit committed to providing clean, safe drinking water to those who suffer needlessly without it.  Nearly one billion people worldwide lack access to safe drinking water. Dirty water causes illnesses such as diarrhea and typhoid. Diarrhea alone kills 1.8 million children each year. Unsafe water causes a cycle of sickness and poverty and communities struggle to teach their children, grow food, go to school and earn a living.  This Christmas season Bad Weasel Games has pledged to give something back.  Help us reach our goal of financing several clean water systems this year.  Check out The Water Project at

As we get more information about our specific water project and how the funds are being applied we will update this page.

This guide can be found at:

We are in the process of converting this guide.  It should be completed by tomorrow.  Thanks for your paitence.

Note that some of the images in this guide are from Big Bad Sudoku Book because Snow Day Sudoku is part of the Sudoku Book family of games.  But the concepts are the same.

Instruction Guide

Snow Day Sudoku is a number placement game of logic.  Sudoku requires no math.  A simple definition of the word Sudoku is "one of each".  The object is to fill in all the blank cells with the correct numbers following these three simple rules:
A sudoku game begins with several cells already filled in.  We call these starting numbers "givens".  Traditional thinking is that the more givens you start with the easier the puzzle will be.  However this isn't always the case.  In Snow Day Sudoku the difficulty of the puzzle is determined by the types of solving techniques required to finish the puzzle.

Here's an example of a simple row in Sudoku:

To solve this line simply think about which of the numbers between 1 and 9 are missing from the line. The answer is obviously 6.  Usually to begin with there will be more than one number missing, but there are always enough clues to help you solve the puzzle.

As you fill in the board with correct answers, the remaining empty cells become easier and easier to solve.  Later in this guide we will go over several basic and advanced techniques for solving puzzles.  A valid sudoku puzzle only has one correct solution.

Snow Day Sudoku is a universal application that can be installed on your iPhone, iPod Touch, and iPad.  This version of the documentation is for the iPhone and iPod Touch.  For instructions on using the iPad version go to:

Quick Start

If you're already familiar with how to play Sudoku, here is how to interact with the Snow Day Sudoku.  If you need a more detailed information you should also read over the Glossary and check out the Solving Sudoku section.

One key difference between Snow Day Sudoku and other sudoku games is that it includes an advanced way of highlighting cells to help solve harder puzzles.  These highlights can be confusing to first time players of our game because they are exclusive to Snow Day Sudoku and other Sudoku Book games.  The thing to remember about highlights is that they don't effect the game in any way other than to help 'highlight' areas of the gameboard.  You don't have to use highlights, however, once you figure out how powerful they are you won't go back.

Highlighted cells are a light blue color. We'll go over all the things that you can do with highlights later in this guide.  For now just know that you can clear all the highlights by tapping outside of the game board.

To enter a number in a cell, select it by tapping once on the cell.  The selected cell is a brighter blue color with an icy border...

Any time a cell is selected the Ornament Box appears and you can tap a number to enter a value in that cell.

To remove the Ornament Box from the screen tap any where that is off the gameboard.  This will also deselect the Selected Cell and in some cases will reveal the status area and icons underneath.  One easy place to tap to remove the Ornament Box is the title Snow Day Sudoku at the top of the screen.

Basic game play is tapping empty cells and entering in the missing numbers.  When the whole board is filled in if all the numbers are in the correct spots it will be Solved!


Cell - One of the individual 91 squares that can contain a number.

Row - A horizontal unit of 9 cells.  A completed row has exactly one instance of each of the numbers 1 through 9.  For a row to be correct each number has to be in the correct position for that puzzle.  In a solved sudoku each row has a unique arrangement of the numbers.

Column - A vertical unit of 9 cells.  A completed column has exactly one instance of each of the numbers 1 through 9.  For a column to be correct each number has to be in the correct position for that puzzle.  In a solved sudoku each column has a unique arrangement of the numbers.

Game Board - 91 Cells arranged in a grid of 9 rows by 9 columns.

Block - A 3 x 3 cell subsection of the Game Board.  On the game board the blocks are emphasized by the thicker grid lines.  Again, a completed block has exactly one instance of each of the numbers 1 through 9.  For a block to be correct each number has to be in the correct position for that puzzle.  In a solved sudoku each block has a unique arrangement of the numbers.

Unit - A row, column or block.  When learning solving techniques it is helpful to understand the concept of a Unit.  Another way to explain the rules of Sudoku is to combine the three rules into one and say that “each Unit must include exactly one of each of the numbers 1 through 9”.  The point is that any solving technique that applies to rows also applies to columns and blocks, and vice-versa.

Givens - A puzzle's starting numbers.  Cells containing givens can not be changed.  In Big Bad Sudoku Book givens are shown in black.

Hints - Snow Day Sudoku has 4 types of hints: Found A Mistake, Found A Single, Found A Hidden Single.  Each hint type will be explained in more detail later in this guide.  Getting a hint negatively effects your score. Read more about hints.

Marks - Also knows as pencil marks.  Marks are small numbers placed in a cell to help keep track of what values that cell could or could not be (candidates). Typically players will set whatever marks could be the answer for that cell, and then narrow them down based on several solving techniques. When there is only one mark left it should be the answer.  Read more about marks.

My Marks vs True Marks - Snow Day Sudoku keeps track of two independent sets of marks.  My Marks are marks that you enter in and can edit. Your marks could be accurate or not, because you can set them however you want. They are shown in blue like the ones in the picture above. True Marks are a computer calculated set of marks that are always correct.  Read more about marks.

Score - Snow Day Sudoku uses the same scoring system as Big Bad Sudoku Book, which awards points for each answer you enter.  Point values are weighed by the difficulty of the answer at the time you entered it, and by the number of remaining possibilities for that cell.  Score is not based on time.  Read more about scoring.

Starting New Puzzles

To start a new puzzle at any time press the plus icon on the lower right of the screen.  Note that when you select a new puzzle the current puzzle, whether in progress or competed, will be erased.  If you want to save the current puzzle in progress and start a new game use profiles.

When you tap the new puzzle icon you'll be given the choice of 5 difficulty ratings.  Tap a card to see puzzles to choose from in that category or tap off the cards to dismiss the new puzzle picker.

The difficulty ratings in Snow Day Sudoku are based on the solving techniques you will needed to use to answer the puzzle.  To learn more about the different solving techniques and what they mean check out the Solving Sudoku section.

Easy puzzles have 33-37 givens and should be singles only.

Medium puzzles have 31-34 givens and could have up to 6 hidden singles.

Hard puzzles have 24-34 givens, 6-17 hidden singles, and will have occassional naked pairs and hidden pairs.

Expert puzzles have at least 5 hidden singles, at least 2 naked pairs or hidden pairs, and up to 2 high level problems such as pointing pairs or box/line intersections.

Insane puzzles are very hard to solve and have at least 2 high level problems and can have any number of the lower level problems.  They do not require guessing but will require the more complex solving techniques.

After picking a difficulty card you'll be shown several game cards to choose from:

Tap a game card to load that game and start playing it.  Tap MORE to see four more games in that level. Tap the backgound or the difficulty card to cancel and remove the new game picker.

Random vs Pattern puzzles - All puzzles are generated randomly.  However, in random puzzles the givens are placed randomly around the game board, while in pattern puzzles the givens follow a symetrical pattern. We provide both types to offer some variation in puzzles and to make the game more fun. Sometimes in random puzzles the givens might be heavy on one side or, like in the example above, you might find a block that is amlost completely full while another one is completely empty. This is part of the puzzle being random and can still be fun to play. With out visual game picker you can alway tap MORE until you find a puzzle that looks interesting.


Highlights are an exclusive feature of Snow Day Sudoku and Big Bad Sudoku Book that can be used to help focus your view to a subset of the game board. Highlights visualize what your brain is already doing when you attempt to solve a sudoku puzzle.  Most of the time you are analyizing the board and looking at rows, columns and blocks for missing numbers.  Highlights are also an easy way to momentarily store a deduction that you've made.

Highlights can be cleared by tapping outside the game board.  An easy place to tap on the small iPhone screen to clear Highlights is the Snow Day Sudoky title at the top.

There are several automatic highlighting features that can be accessed through touch gestures. We think that once you use highlights you'll never go back.

Highlight Units (Long Tap Empty Cell)

Tapping on an empty cell selects it so that you can enter in a value. Tapping and holding on an empty cell, or the long tap, selects it and also highlights all 3 units associated with that cell: the row, column and block that it's in.

This helps focus your attention to the adjoining cells that matter for that cell. In the example above we can look at all of the highligted cells and look at what numbers are missing.  The answer is 7.

Highlight All Numbers  (Long Tap A Number)

Tapping and holding on a number, whether a given or one of your answers, will highlight all copies of that number in the game board.

This makes it easy to see what units are missing that number.

Highlight All Units for All Numbers (Long Tap A Number Twice)

Tap and hold the number a second time and it will highlight every unit for every copy of that number.  This shows every cell that the number CAN'T be in.  This feature is almost too good and some people consider it cheating.  But there are times when it comes in handy.

In the example above it shows where the 3 remaining 8's go.  Using this feature does negatively effect your score.  However if you do it manually it does not effect your score.  Here's how to do it yourself.

Highlight A Row or Column (Swipe In To A Row or Column)

Swipe (or drag your finger) in to a row or column from outside the game board and it will highlight that row or column. These swipes are additive and won't de-highlight anything that was already highlighted.

Combine these swipes with the Highlight All Numbers explained above and you can easily get to the same conclusion as the Highlight All Units for All Numbers, except that you did the work. There is no score penalty when you do the work.

Custom Highlight Cells (Draw on the Game Board)

The final highlighting feature is simply drawing on the game board.  This type of highlighting is very useful in analying the complex chains that can be found in Insane puzzles.

Using Marks

Some people call them Pencil Marks but we just call them Marks.  Marks are small numbers or tick marks that are placed in the corners of a cell to indicate that it is still open as a possible answer for that cell.

To use your own Marks in Snow Day Sudoku first make sure that Show My Marks has been turned on inside of Marks Options.

When you Select an empty cell, if you have Show My Marks turned on you'll see the Marks Entry Box:

Think of the Marks entry box as a ZOOM-IN or close up of the selected cell.  It shows a close up of cell so you can see the marks for that cell more easily.  By tapping on the numbers you will toggle that mark on and off.  There are also 3 icons below the marks entry box:

For an example of how to use My Marks to solve puzzles lets look at the following single row:

Knowing only this information about the empty cells we can deduce that the missing numbers on the line are 1, 4, and 9. But for now we don't know which cell contains which number. So each number is a candidate for each cell. Marks help you keep of these candidates. As we solve other cells that relate to these, we get more information about what CAN NOT be in these cells, and we can remove them as candidates by removing or turning off the marks.

To begin with we will put those 3 marks in those 3 cells:

Of course a real sudoku game also has colums and blocks and you use all these related cells to gather information. If we expand our view to include all 3 blocks surrounding this row we can see other numbers in the mix and remove some of the candidates:


Remember that there can only be one instance of each number in each row column and block.  The 4 in the first block removes it as a candidate for all empty cells in that block and therefore leaves only the 1 and 9.  In the next block the 1 removes it as a candidate.  And in the last cell the 9 and the 4 are removed as candidates.  When only one candidate remains it must be the answer for that cell.

In this example we now can solve the whole line. The 1 is the only remaining candidate for the last spot.  That is called a Single because there is only one candidate left.  Once we solve the 1, the 1 can be removed as a candidate for the first spot leaving only the 9, and in turn this leaves only the 4 for the middle spot.

When using My Marks you have to do all of the adding and removing of marks yourself.  If the computer did it for you it would be a serious advantage.  However, that's exactly what True Marks are.

True Marks

What True Marks do is actively update marks across the whole game board as you enter values in related cells.  In the example above the True Marks would automatically remove the 1 from that first spot when you entered one as an answer to the last spot. The only caveat with True Marks is that if you enter in a wrong answer the marks will follow that. They don't guarentee that you're answering cells correctly.  If you ever see True Marks show NO markes in a cell, you've answered something incorrectly somewhere and you should use Get Hint to find out where.

Be aware that using True Marks will negatively effect your score.  Using My Marks does not.

One thing you can't do with True Marks is edit them. Why would you want to edit them if they're right? True Marks only takes the basic approach of looking at all 3 units to remove candidates for a cell.  This method will help solve Easy Puzzles but to solve harder puzzles you have to use more advanced techniques to narrow down your candidates.

So we included an option to copy the True Marks to My Marks where they can be edited.

Marks Options

Tap the Marks Options icon to bring up the Marks Option card. From there you can switch between True Marks and My Marks, and do mass modification to your marks.  Here is the card and what each option does:

Marks Display Modes - The marks icons will be darker when they are being displayed and lighter when they are turned off.  Only one set of marks can be displayed at a time.  Turning on one set will turn off the other.  Remember that any time you manually enter a My Mark it will turn them on so you can see the mark you just entered.

Reveal / Hide True Marks - Turns on the True Marks display and turns off the My Marks display if they were on.  Turning on True Marks will cause a penalty in the score for the next cell you answer and will reduce your bonus points when you solve the puzzle.

Show / Hide My Marks - Turns on and off the custom marks.  There is no penalty for using My Marks.

Copy True Marks to My Marks - takes a snapshot of the current state of the True Marks and copies them to your My Marks.  This will erase any previous marks you had made.  This option will cause a one time penaly in the score for the next cell you answer and will reduce your bonus points when you solve the puzzle.

Clear All My Marks - resets all of your custom My Marks by turning them all off.

Set All My Marks On - turns all 9 marks on for all empty cells.  You might want to do this to start with them all on and remove them as you go.

Get Hint

The compass icon is the Get Hint feature. Get Hint does exactly that, it gives you a hint without giving you any answers. To understand the hints you will need to understand a couple of basic solving techniques.  These are explained in the Solving Sudoku section. Get Hint offers three different kinds of hints: Found A Mistake, Found A Single, and Found A Hidden Single. The hints are tiered, meaning that you'll only get a hidden single if there are no singles to be found, and only get a single if there are no mistakes.  Each use of Get Hint only gives you one hint for one cell.  Repeated use can result in the same hint being given over and over, or cycle through several hints, depending on the state of the board.  Using Get Hint results in a penalty for the next cell you answer.

Found A Mistake (Red Highlight) - Will show a cell that contains a wrong answer by highlighting it Red.

Found A Single (Green Highlight) - Will show a cell that contains a naked single by highlighting it Green.

Found A Hidden Single (Teal Highlight) - Shows a cell that contains a hidden single by highlighting it a Teal Color.

Game Options

Snow Day Sudoku has several game options to customize your playing experince. To access the Game Options Card tap the Game Options Icon.

Sound Effects - Whether or not to play sounds during the game.

Line Completion Sparkle: When units (rows, columns, or blocks) are completed a sparkle animates across them. This options customizes the conditions for the sparkle to animate.

Always - The sparkle animates when ever all 9 cells are filled in, whether the answers are correct or not.  In "Always" even if you put the same number in each cell it will still animate.

1-9 - The sparkle animates only if you don't have any repeated numbers.  You have to have each of the numbers 1 through 9.  However, in this mode the sparkle will still animate if you have 2 of the numbers swapped.

Only If Correct - The sparkle animates only if all 9 numbers are in the correct spots.

Why so much attention to the sparkle animation?  The 3 different options offer different levels of verification that your answers are correct. Some players don't like any spoilers.  If you have the options set to Only If Correct and you fill in a row but there's no sparkle, you would know that you had made a mistake.  If you don't want this kind of information we provide the option to set it to Always or 1-9.

Timer Clock Display - Personally I don't like playing against a clock, so we put in the option to not see the clock until you've completed your game.

Score Display - This options also falls in the "spoilers" category.  Since the score is negative when you answer the cell incorrectly, the score is actually a huge hint.  If you don't want to see these hints, or if you just don't like the score you have the option of turning it to only show the Score when you've solved the puzzle.  FYI - It is harder to get a good score with it turned off.

Game Center Alias - If you are under iOS 4.1 or later, Game Center is enabled and this option will show your Game Center Alias.


Snow Day Sudoku employes an exclusive method of scoring games. The idea behind our scoring system is that you get points for each cell that you answer, plus you get a bonus for correctly completing units, and you get a bonus for completing the puzzle.  The score for answering a cell is determined by the difficulty of the answer and by how many candidates were left for the cell when you answered it.

The math behind the scoring system is a little involved because it considers previous wrong answers, the type of method required to solve the cell, and the number of candidates.  It also consideres if the player used any hints or true marks.  Here are the basics.

Cell Points

If True Marks or the double long tap were used to help answer the cell there is a 9 point penalty.

Line Completion Bonus Points

A line completion is an easy way to say a unit completion - so it's awarded when a row, column or block is completely filled in correctly.  Multiple units completed at the same time multiplies the bonus.
You only get the line completion bonus if you completed the line correctly the first time. An incorrect answer in the line forfits the bonus for that line.  You can only get the bonus once per unit.

Puzzle Completion Bonus

There is a potiential of 18 bonus points per cell.  Using any hints, true marks, or double long tap highlighting (cheats) or getting a wrong answer wipes the bonus for that cell.

At the end of the game the bonus is tallied. 10% is taken off for each hint that was used during the game.  Another 25% is removed if true marks were ever used during the game.  It is possible to lose your entire puzzle completion bonus by using too many hints and true marks.

Game Center

Snow Day Sudoku uses Game Center to track Leaderboards and Achievements.  For competitive play try using Puzzle Sharing.  Game Center requires iOS 4.1 or later, an internet or cellular connection, and a Game Center account.

The easiest way to use Game Center is to log in using the Game Center app, then launch Snow Day Sudoku.  You should only ever have to log in to Game Center once (or once in a while) on each device.

If you're device supports Game Center and you launch Snow Day Sudoku without first logging in to Game Center, you will be asked to log in.  At this point if you hit "Cancel" Sudoku Book will assume that you don't wish to use Game Center and it won't bother you with the log in screen again.

The two icons above are the Leaderboards icon and the Achievements Icon.  When you are logged in to Game Center these icons will be fully black (active). When you are logged out of Game Center they will be grey or dehighlighted.

When they are active if you tap them you'll be taken to Apple's Game Center interface where you can browse the Leaderboard or see your Achievements.  When they are dehighlighted tapping one of the icons will give you the opportunity to log back in.

Note that our implimentation of Game Center is broken right now and we expect it to be functioning correctly in a day or two.


Snow Day Sudoku keeps track of 7 Leaderboards using Game Center.  They are:


Snow Day Sudoku has 9 Achievements that you can earn.  Some of them are hidden until you complete other Achievements (and we won't spoil them here).  The initial ones are:

Solving Sudoku

In the Using Marks section of this guide we went over the basic idea of keeping track of all the possible candidates for each cell by using My Marks. This technique is critical to solving harder puzzles and is a prerequsite for many of the solving methods we will discuss here.  It's a good idea to read over that section before jumping into this one.

There are many different ways to approach solving a sudoku puzzle.  Here we will go over a few popular methods as breifly as possible and also describe some of the solving terms such as Single, Hidden Single, Naked Pairs, Hidden Pairs, and others.

A great first concept to grasp is the idea of the Unit. A Unit is either a row, a column, or a 3x3 block. Check out the Glossary for more in depth descriptions of each of those.

Any rule or solving concept that can be applied to one type of unit can be applied to another type. For example, the basic rule of Sudoku is that each unit must contain exactly one instance of each of the numbers 1-9. If you look at any of the 81 cells on the board you can see that it is a member of 3 units.  It is in a row, it is in a column, and it is in a block.  So when considering any particular cell and what number could go in that cell, you have to look at each of the 3 units that cell is in and eliminate any number as a possibilty that is already being used somewhere else in any related unit.

That might be a complicated way of saying what can easily be shown by long tapping an empty cell:

Each of the highlighted cells above are related to the selected one.  Any number that is in any of the highlighted cells can not be the answer for the selected cell.


In the example above, it is clear that the only remaining candidate is a 7.  When a cell only has one remaining possible candidate it is called a Single.  It is sometimes referred to as a Naked Single because the candidate is sitting out there all by itself.

The easiest way to find singles without using True Marks is to use the long tap to highlight the units related to a cell and cross-scan for them.

Hidden Singles

A hidden single is when a particular candidate can only be found in one cell of a unit.  For example if a row has several empty cells but only one of the cells has a 1 as a candidate, the 1 must go there. To demonstrate the hidden single concept let's look at a fresh Insane puzzle:

In the puzzle above a cross-scan reveals that the selected cell has 3 candidates: 5, 6 and 7. Looking first at only the row you'll find that all 3 of the candidates can be found as candidates in other cells. Next look at the column. Again the 5, 6 and 7 are repeated several times as candidates in other cells in the column. However if you look at only the block, the 6 and 7 appear in other cells but the 5 is exclusive to that cell.  This exclusitivity makes it a Hidden Single and 5 has to be the answer for that cell.

Looking at all the highlighted cells above do you see any other hidden singles?  The sixth cell over in the row has the candidates 1 and 7.  Since that's the only cell in the row that can be a 1 it must be a 1.

Another way to find hidden singles is to use the double long tap.

Advanced Solving Techniques

Singles and Hidden Singles are concepts that lead to a direct answer for a cell. If the cell contains a single or hidden single then that's the answer. Advanced solving techniques are ways of using logic to reduce candidates even further. The more you reduce candidates across the puzzle, the more singles and hidden singles you'll find.

Also, any time you solve a cell you can remove the answer as a candidate for all of its related cells.  Solving Sudoku is as simple as repeating the cycle of finding singles and hidden singles, reducing candidates, and finding singles and hidden singles again, and repeat.  This is how computers solve sudoku.

Here are a few advanced techniques.

Naked Pairs

If you have two cells in a unit that each contain an identical pair of candidates and only those two candidates, then no other cells in that unit can be those two values.

Think of it this way: if A has to be either a 1 or a 2 and B also has to be either a 1 or a 2, then one of them has to be a 1 and the other has to be a 2.  You have two slots for two numbers. Therefore no other cells in that unit could be 1 or 2 because they're already taken up by those two cells. Let's visualize using the same Insane puzzle:

In this example every cell has 7 as a candidate. But the bottom two cells form a naked pair because of ther matching candidates. Both have 7 and 2. No matter what's going on in the other cells we know that one of those two cells must be a 7 and the other one must be a 2.  Therefore no other cell in the group can be a 2 or a 7.  We can eliminate the 7 as a candidate for the other two cells. When we do this it leaves only 6 as a Single in the top cell.  This revelation further reduces the candidates in the middle cell only leaving the 5 - which is the same answer we reached in the Hidden Single exercise above.

Naked Triples

The same concept can be applied to triples and quads.  If you have 3 cells each with only the same 3 candidates, those 3 candidates have to go in those 3 cells and therefore can't go anywhere else in the unit.

Here, from the same Insane puzzle, we have 3 cells each with only the candidates 4, 6, and 7.  These 3 cells must contain these 3 numbers, although at this point we don't know which cell contains which number. But the remaining 3 cells are instantly solved by this method.  The 7 must be removed as a possibility from the rest of the block. This leaves the 1, 9 and 8 as Singles.

When you get into triples and quads the rule even gets relaxed a little bit. As long as the 3 cells don't contain any extra candidates, they don't all 3 have to contain all 3. Because at the end of the day you still have 3 cells which have to contain 3 candidates, so all 3 of those candidates can be removed from the rest of the unit.  Look at this example:

Three cells contain the combined candidates of 3, 4 and 7. Those 3 candidates can be removed from the rest of the row:

In this case reducing the candidates didn't directly reveal any singles. But it did reveal another Naked Pair.  The 2-5 2-5 is a pair which removes the 2 from the 2-9 leaving only the 9 as a Single.  Another way to look at it is that the candidate reduction revealed the 9 as a new Hidden Single.

Hidden Pairs

The hidden pair takes the logic of a hidden single and applies it to pairs. If two cells in a unit contain a pair of candidates, hidden among other candidates, that are not found in any other cells in the unit, the two candidates have to go in those two cells.  This one is certainly easier to show:

This candidates in this block appear to be all over the place. But on close inspection you'll notice that the 7 and 9 only appear in two cells. This means that the 7 is in one of those two cells and the 9 is also in one of those to cells. So again you have two cells that must contain the two candidates.  It's still a pair but it was a little harder to find.  You can remove the rest of the candidates from these two cells.

What does this candidate reduction reveal about the rest of the cells in the block?  Well, it allows you to easily solve the two highlighted cells. Choose your solving method. The cell on the right is left with a 6 as a hidden single.  The 2-5 2-5 is another naked pair which eliminates the 5 from the cell on the left, leaving only the 3 as a single there.

Pointing Pairs

When a particular candidate only occurs in one row within a block, it can be eliminated from the rest of the row.  Same for when a candidate only occurs in one column within a block, it can be eliminated from the rest of the column.

In the example above the two highlighted cells to the right are the only two cells in that block that contain the canidate 5. The 5 has to be in one of those two cells. Since those two cells are in the same row inside that block, the 5 can't be anywhere else in that row and therefore can't be in the highlighted cell on the left. Eliminating 5 as a candidate from the cell on the left reveals a hidden single in the 5 above it.

More Solving Methods

If you're looking for more techniques google "Sudoku Box Line Reduction"  "Sudoku X-Wing"  and "Sudoku Chains".

More Help

If there is anything this document doesn't cover you can email us question, comments and feature requests at

Known Issues

This guide is current for version 1.0.1 of Snow Day Sudoku.  There are no known issues at this time.