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.

An = between two cells says they hold the same; an × says they do not.

Tap a cell for a filled square, tap again for a hollow one, once more to clear it. The squares the board came with stay put: tap one and they all give a shake, to show which they are.

Shown in red: three of the same kind side by side, squares a row or column has too many of, and two cells that disagree with the sign between them.

Learn the solving techniques