Sudoku X builds on the rules of Sudoku. If you don't know how to play Sudoku then take a look first at the rules of Sudoku.

A Sudoku X puzzle adds to the standard rows, columns and bold-lined boxes by also requiring that you place 1 to 9 (or 1 to whatever the width of the puzzle is) into each of the two main diagonals, highlighted on puzzlemix in pink:

In this example puzzle you can use this new rule to immediately place the '1' in the centre box, as shown in the picture below. It can be eliminated from the centre square of the box because it is already in one of the diagonals.

When the puzzle is fully-solved you will have 1 to 9 in each row, column, 3-by-3 box and marked diagonal as shown here:

And that's all there is to Sudoku X!