Sudoku DVD game auto gym HDTV sun photo wine space yachts BBQ baths astro phones spy GPS iPods PC gadget discussion forums Sudoku puzzles now appear daily in newspapers around the world, rivaling crosswords for popularity. They are usually introduced with these instructions: Complete the grid so that every row, column and every three-by-three box contains the digits 1 to 9. Solve the puzzle by logic and reasoning alone, there is no maths involved. If anything this description shows up the general lack of knowledge about what maths is. Maths is precisely "logic and reasoning". What they mean is that sudoku requires no arithmetic. In fact Sudoku is an excellent way to get a feel for what it is like to do mathematics for real and is great for increasing children's interest in the subject, especially if you don't make the mistake of telling them that sudoku is not maths! With the rant over, here is a list of common sudoku terminology in use. Many terms and rules in sudoku have more than one name. That is simply because people have discovered them independently many times over and use different terms. For example a box is often called a block. I have tried to give the most current and consistent term that I know of, but have noted some popular alternatives in brackets. Often I have used a Google search to vote for the term used the most. I have expressed the terms for the standard Sudoku 9 by 9 grid with digits as the symbols. They can of course by applied to grids of other sizes but I wanted to make this a readable reference rather than a precise and general set of mathematical definitions. Sudoku Grid Terms Band (floor): Three boxes connected horizontally to make a 9 by 3 rectangle. Box (block): One of the 3 by 3 squares of cells in the grid when it is divided into nine equal squares. Cell (square): A single square in the puzzle where one digit can be placed Chute: A band or a stack Column: 9 cells of the puzzle running in a straight line from top to bottom Grid: The full 9 by 9 set of cells Group (house, unit, scope): A box, row or column Line: A box or column Pair: Two cells in the same group with only the same two candidates Rectangle: This can refer to any rectangular set of cells of any size. Row: 9 cells of the puzzle running in a straight line from left to right Square: This can refer to any square set of cells of any size. Stack (tower): Three boxes stacked vertically to make a 3 by 9 rectangle. Other General Terms Candidate: A digit that has not been eliminated from a cell Conjugate Pair: Two cells in a group which are the only cells in the group where one particular digit can go Digit: a whole number from 1 to 9 Elimination: Using rules to reduce the number of possible digits that can be placed in a cell Placing: Entering a digit into a cell Rule: A logical method to place a number in the puzzle or eliminate the candidates Single: A cell with only one candidate Strategy: A method that can be used to find solutions quickly or efficiently Triple: Three cells in the same group with only two or three candidates each and only three different candidates between them Rules Box interaction: When two blocks are aligned on the same three rows or columns and a digit is eliminated from all but the same two rows (or columns) in both boxes, it can be eliminated from the other cells in those two rows (or columns) Hidden subset: When n digits have been eliminated from all but n cells in a group, all other digits can be eliminated from those cells Hidden pair: When two digits have been eliminated from all but two cells in a group all other digits can be eliminated from those cells Hidden quad: When four digits have been eliminated from all but four cells in a group all other digits can be eliminated from those cells Hidden single: A digit can be placed when it is eliminated from all but one cell in a group Hidden triple: When three digits have been eliminated from all but three cells in a group all other digits can be eliminated from those cells Locked candidates: When a digit in a box is restricted to the intersection with a column or row it can be eliminated from the other cells in the column or row. Naked pair: When two cells in a group can only contain the same two digits, those digits can be eliminated from all other cells in the group Naked quad: When four cells in a group can only contain the same four digits, those digits can be eliminated from all other cells in the group Naked single: When a digit is known in a cell it can be eliminated from all other cells in groups containing that cell Naked subset: When all the cells in a subset of n cells in a group can only contain the same n digits, those digits can be eliminated from all other cells in the group Naked triple: When three cells in a group have only the same three candidates, those digits can be eliminated from all other cells in the group Reverse locked candidates: When a digit in a row or column is restricted to the intersection with a box it can be eliminated from the other cells in the box. Swordfish: When a digit appears in three conjugate pairs in three rows and the digits are aligned in pairs in three columns, the digit can be eliminated from the other cells in the three columns that contain them. The same rules applied when rows and columns are interchanged. X-wing: When a digit forms two conjugate pairs in two rows and the four cells of the conjugate pairs are aligned to form a rectangle, the digit can be eliminated from the other cells in the two columns that contain them. The same rules applied when rows and columns are interchanged. Strategies Bifurcation: Select a cell with two candidates and proceed by trial and error. If the selected candidate is eliminated you can place the other Cell scanning: Pick a cell and look at the digits in the 20 other cells that it shares a group with. Eliminate those digits from the cell Mark up: Writing the candidate lists in each cell Nishio: consider placing a given number in a cell and determine if that prevents it bring entered somewhere for each block, row or column Trial and Error: Select a candidate digit and consider the effect of placing it. Continue to see if the puzzle can be completed. If it cannot then that digit can be eliminated from the cell. If it can, you have completed the puzzle.