Number Square

Tap a square to step through the digits and once more to clear it, or type the digit you want. Shaded squares are fixed and will not change.

Each row holds every digit once, and so does each column. Place a second copy anywhere along either and both squares turn red until one of them changes.

The heavier lines divide the grid into boxes — two by two on the small board, three across and two down on the large one. Each box holds every digit once, which is the rule a row-and-column check alone will not catch.