Hidden single

A hidden single is a number that has only one possible cell in a row, column or box, even if that cell could also hold other numbers. It’s “hidden” because the cell itself doesn’t look forced.

How to spot it

1. Choose a box, row or column and a number it’s still missing.
2. For each empty cell in it, check whether that number is blocked by a copy in the cell’s row or column (or box).
3. If only one cell remains, the number goes there.

A real example

In box 5, every empty cell except r5c5 already sees a 6 in its row or column (the highlighted 6s). The crossed-out marks show where 6 can’t go, so it must go in r5c5.

Tips

• Scan for a number that appears often on the board. Its copies block many cells at once.
• Do this for boxes first. Two copies of a number in neighbouring rows usually leave a box with one or two spots.

Practice on Easy puzzles