Sudoku Rules and Tips

Sudoku is a Japanese puzzle where you have nine 3 x 3 blocks where you have to enter the numbers 1 to 9. The rules are that Vertical and Horizontal numbers cannot repeat and each number can only appear once in each 3 x 3 cell. Some numbers are fixed at the start of the game and you just have to complete the rest of the numbers to finish the puzzle. The number of empty cells and numbers depends on how challenging you want the puzzle to be and the higher the challenge level the fewer cells are filled in for you at the start. At the harder challenge levels you need to use several specific techniques to fill the entire grid and finish the puzzle.

The first step in solving a Sudoku puzzle is to check all blank cells for any possible intersecting numbers and by observation ink in any numbers that can only be positioned in one particular cell. Once you get stuck and you cannot see any more solutions for any cells by observation your next move is to pencil in all possible values in all the empty cells. These pencil notes are all the possible candidate numbers for the correct number in each cell and by a process of elimination we use these notes to solve numbers for each cell.

Once you have all candidate notes in all the empty cells then you have a clear view of the board and you can sit back and use the notes to eliminate candidates. There are several techniques you can use to eliminate candidates and by updating your notes you will see the correct numbers for cells. To solve Level 4 puzzles you will need to use all of the techniques described below.

By clicking on the 'Show candidates' button in this App you can see all the possible candidates for all empty cells. The candidates are shown from the position when you first click the button and after you have entered new numbers into a cell the candidates will not automatically update. To force a refresh and see the updated candidates please enter and then delete a wrong number into any empty cell eg enter a 2 beside an existing 2 and then delete the wrong 2. This will update and refresh the remaining candidates for all the empty cells.

1) Naked Singles
These are candidates that are the only candidates in a cell eg 3 or 5. Ink those in and thank your lucky stars for each one you find because to find all of the other numbers you have to laboriously and logically eliminate candidate notes until there is only 1 possible candidate left in a cell.

2) Naked Pairs
These occur when two cells within the same house (a 3x3 block, row or column) contain only the exact same two candidate notes eg 26 and 26. This means that 2 and 6 must exist in one of these 2 cells and we can therefore eliminate all other 2 and 6 candidates from all other cells in that house. Eliminating candidates may lead to the exposure of more Obvious Singles.

3) Naked Triples
These occur less often than Pairs but follow the same rules but apply to 3 cells all holding the same 3 notes eg 128, 128 and 128. Once identified we can eliminate those 3 values from all other cells in that house.

4) Hidden Singles
These occur when a cell contains a unique number that does not occur in any other cell in that house eg 138 is in a cell and the 8 does not occur in any other cell in that house. Therefore 8 is a Hidden Single.

5) Hidden Pairs
These occur when two numbers (e.g 2 and 4) appear as candidates only in two cells in a house but those cells also contain other notes (e.g 2, 3, 4, 6, 8 and 2, 4, 6, 7). Therefore we can eliminate numbers other than 2 and 4 from those 2 cells.

6) Hidden Triples
These occur when three numbers (e.g 2, 3 and 4) appear as candidates only in three cells in a house but those cells also contain other notes (e.g 2, 3, 4, 6, 8 and 2, 4, 6, 7). Therefore we can eliminate numbers other than 2, 3 and 4 from those 2 cells.

7) Pointing Pairs
These occur when a candidate appears only two or three times within a single 3x3 block and they are all in the same row or column. Logically that number must be the solution for one of those cells inside that block so it points along the line so you can eliminate that candidate from any other cells in the rest of the row or column outside of that specific block.

To solve Sudoku puzzles at the higher challenge levels you will need to apply these techniques in a rotating sequence (ie more than once) until you have eliminated enough candidates to have found numbers for each cell.

2026 Steve Wright