Basic

Hidden single

What it is

A digit has only one possible place in a box, row or column.

How to spot it

Pick a digit and a box (or row or column) that is missing it. Cross out every cell that already sees that digit in its row, column or box – if only one empty cell is left, the digit goes there.

Example

Basis
Removed / placed

Box 1 is missing a 5. The 5s in r2c5 and r3c8 rule out rows 2 and 3, and r1c2 and r1c3 are already filled, so the 5 must go in r1c1.

Used from level: Extra easy