Hidden Single (row or column)

If a digit has only one possible cell left in a row or column, it goes in that cell.

Level: Elementary · technique 2 of 7

When to use it

Use it once the box-by-box search dries up. It shows best in a row or column with only three or four empty cells.

A row or column crosses three boxes, so a cell can be blocked by a box as well as by a crossing line. That is why these are harder to spot than hidden singles in a box.

How to find it

1. Pick a row or column with few empty cells.
2. Pick a digit that line is still missing.
3. For each empty cell of the line, check whether its crossing column (or row) or its box already holds the digit, and cross it off if so.
4. If only one empty cell survives, that is the answer.

Worked example

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

1. Row 4 has no 4 yet. Its empty cells are r4c1, r4c6, r4c7.
2. Every other empty cell is blocked by a 4 already in a crossing line or in its box: r4c1 (the 4 in column 1), r4c6 (the 4 in column 6).
3. Only r4c7 is left, so r4c7 is 4.

Common mistakes

• Checking only the crossing lines and forgetting the boxes: if a cell's box already has the digit, that cell is blocked too.
• When a line is missing several digits, check them one at a time. Looking at all of them at once is where it goes wrong.