Every row and every column holds the digits from 1 up to the number of its cells, each one once — on a 5 × 5 that is 1 to 5.

A sign between two cells points at the smaller of the two digits: 2 < 5, and 5 > 2. Between a cell and the one below it, it reads the same way.

A few digits come with the board; the signs carry the rest.

Tap a cell and pick a digit on the pad under the board; the same digit again clears it. With notes on, digits are pencilled small in the cell instead. The digits the board came with stay put, and the signs take no taps — a tap on one lands on the cell under your finger.

A reddish cell holds a digit its row or column already has, or a digit on the wrong side of a sign — and that sign reddens with it.

Learn the solving techniques