Fill every cell with a 0 or a 1 (or white and black circles, if you prefer the classic look). Tap a cell to cycle: empty → 0 → 1 → empty.

No three in a row: three identical digits may never sit consecutively, horizontally or vertically. Two is fine — a pair always gets capped by the opposite digit.

Balance every line: each row and each column contains exactly as many 0s as 1s — half and half.

Unique lines: no two rows may be identical, and no two columns may be identical. On hard boards this rule is the key that unlocks the finish.

The board is solved when every cell is filled and all three rules hold. There is always exactly one solution, and it never requires guessing.