Hidden Single (box)

If a digit has only one possible cell left inside a box, it goes in that cell.

Level: Beginner · technique 1 of 7

When to use it

This is the first technique to reach for at the start of a puzzle. Pick a digit that already appears often, then look at each box that is still missing it.

Because it asks where one digit can go rather than what a cell can hold, it works without writing any candidates down. It is often called crosshatching.

How to find it

1. Pick a digit. The more often it already appears on the board, the easier this is.
2. Pick a box that does not contain that digit yet.
3. Extend every row and column holding that digit across the box and cross off the cells they block.
4. If only one empty cell in the box survives, that is where the digit goes.

Worked example

On the board below, find where digit 3 goes in box 4.

1. Box 4 has no 3 yet. Its empty cells are r4c1, r5c1, r5c2, r6c1, r6c2, r6c3.
2. Every other empty cell is blocked by a 3 already in its row or column: r4c1 (the 3 in row 4), r5c1 (the 3 in column 1), r6c1 (the 3 in row 6), r6c2 (the 3 in row 6), r6c3 (the 3 in row 6).
3. Only r5c2 is left, so r5c2 is 3 - even though that cell may still have other candidates.

Common mistakes

• Missing one of the box's empty cells while counting. Count all of them first, then cross off.
• The answer cell can still have other candidates. This technique is about where a digit goes, not about what a cell can hold.
• Check whether the box already contains the digit before you start; if it does, move on to another box.