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The rules of Sudoku are as follows: fill all the empty cells of the puzzle with the numbers 1-9. Each number can only appear once in a row, column or region. As they are made up of 9 cells, each row, column and region must contain all the numbers from 1 to 9. There is only ever one possible solution to each Sudoku puzzle.
To demonstrate this point further, we have divided the Sudoku puzzle up into rows, columns and regions. The rows and the columns are numbered 1-9. The regions are labelled according to their position. You can see these different zones on the Sudoku puzzle opposite (puzzle 1).
One of the most basic methods for completing a Sudoku puzzle involves finding all the possible positions for a given value. You can then note down these numbers at the top of the corresponding cell. The various possible positions for a given value are referred to as ‘options’. The Sudoku puzzle opposite (puzzle 2) clearly illustrates this point.
If you employ Sudoku techniques 5 to 10, you will need to note down all the options, i.e. all the possible positions for all the numbers from 1-9. Otherwise, the technique will not work. To do this, you will need to employ Technique No.1: Using horizontal and vertical clues and Technique No. 4: Cross-checking. We recommend that you make a note of the numbers at the top of the cell, as illustrated above. As you fill in the cells, it is advisable to delete the values you have found from the options you have noted down in all the other cells in the same row, column and region. By narrowing down the options, you are making it easier to complete the rest of the puzzle. Make sure you note down the options in pencil so you can erase them easily. In the examples provided, for the sake of clarity, we will simply put a line through the options as we eliminate them.
The white numbers are those that we had to fill in to allow us to illustrate the particular Sudoku technique. The red numbers are those that we have been able to fill in as a result of using the technique.
General advice
Only ever fill in a cell with a value when it is the only possible solution. Never definitively enter a number if it is only a possibility, as you do not want to risk making a mistake, which will subsequently have an impact on the other values in the puzzle. It is also extremely difficult to find where you have gone wrong once the mistake is made. The fewer empty cells there are in a row, column or region, the greater your chances of being able to find the missing values. You should therefore start with the zones with the most completed values.
Put this technique to the test with one of our online Sudoku puzzles |
