Windoku, Hypersudoku,NRC-Sudoku, or Four-square Sudoku is one of the Sudoku Variations with additional constraints. This format was first introduced in the Dutch newspaper NRC Handelsblad by Peter Ritmeester and the first playable online version by Chris McCusker.
Windoku Rules : (Windoku Example)
1. All rows must have all the numbers from 1 - 9 in them (none can be repeated). There are 9 rows in the game.
2. All columns must have all the numbers from 1 - 9 in them (none can be repeated). There are 9 columns in the game.
3. All 3x3 squares must have all the numbers from 1 - 9 in them (none can be repeated). There are 13 squares in the game. There are the 9 underlaying 3x3 squares (divided by the dark blue lines) and the 4 overlaying (shown in light blue in the following diagrams).