Put a filled or a hollow square in every empty cell so that each row and each column holds as many of one as of the other.

Never three alike side by side — not in a row, not in a column.

Every cell of a block holds the same square.

Tap a cell for a hollow square, tap again for a filled one, once more to clear it.

Shown in red: three of the same kind side by side, squares a row or column has too many of, and a block holding both kinds.

Learn the solving techniques