Fill every empty cell with a hollow square or a filled one, 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.

No two rows read the same, and no two columns do either.

Tap a cell for a filled square, tap again for a hollow 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 two lines that read the same.

Learn the solving techniques