Sudoku Classic is the standard form of the puzzle - one number, from 1 to 9, must be entered into each box such that every row, column and 3X3 square contains each number once and only once. There is only one solution for each puzzle. Some initial numbers are provided within the diagram.

Using the same logic as the classic version, Sudoku Alpha uses a set of nine letters rather than numbers. The available letters are shown below the grid. The initial values provided can include a short phrase constructed from some or all of the given letters, but this doesn't impact on the way the game is played.

A more advanced form of the puzzle - as well as the rows, columns and 3X3 squares other sets of nine boxes are filled in with different colours. These extra sets of nine boxes must also obey the same rule, i.e. the numbers 1 to 9 must appear once and only once in these extra sets as well as all the others.

In this form of the puzzle all the normal rules apply, but in addition there are two 'snakes', one going across from a '1' in the left hand column to a '9' in the right hand column, and one going down from a '1' in the top row to a '9' in the bottom row. Each cell in the snake must be adjacent or diagonal to the preceding one. No cell can be in both snakes. If you enter a number which would be part of the snake if it is in the right row or column, then it is shown in a different colour (green for the vertical snake and pink for the horizontal snake).

An online version of the popular childhood puzzle. Click & drag the blank square over any adjacent tile to rebuild today's Audlem Online homepage picture, which has been randomly rearranged. The extra tile to the right hand side can be exchanged with any of the tiles in the right-hand column.

Test your mental arithmetic. Start with the top number and follow the instructions down the page to work out the answer. Enter the answer in the box at the bottom and click check. Every puzzle is unique and the quicker you do it, the cleverer you are!