Sudoku is a logic puzzle with simple rules. Fill in the 9×9 grid so that the digits 1–9 appear exactly once in each row, column, and box. Fill in the 9×9 grid so that the digits 1–9 appear exactly once in each row, column, and group. Fill in the 9×9 grid so that the digits 1–9 appear exactly once in each row, column, and box (or group, on an irregular board).
Square puzzles have the familiar 3×3 boxes arranged in a regular grid.
Irregular puzzles follow the same rules, but the boxes become groups: jigsaw-shaped regions of 9 cells each. The colored shading shows which cells belong to the same group.
X variants add two extra constraints: both main diagonals must also contain the digits 1–9. The diagonals are shown as lines crossing the grid.
Sudoku X4 offers all four combinations: Square, Square X, Irregular, and Irregular X.
Every puzzle Sudoku X4 makes has exactly one solution, and none requires guessing. A puzzle you import must also have exactly one solution, but it may need guessing.