Beginner

Hidden Single

A digit that fits in only one cell of a row, column, or box — even when that cell still has other candidates.

A hidden single is a digit that has exactly one legal home within a unit. The cell may still carry several pencil marks, so the placement is "hidden" behind those other candidates, but the digit itself has nowhere else to go in that row, column, or box.

Hidden singles are the highest-value beginner technique: most puzzles up to medium are solved almost entirely by alternating naked and hidden singles.

How to spot it

Take one digit at a time and one unit at a time. Within the unit, mark which cells could legally hold that digit. If only one cell can, that cell is a hidden single for the digit — place it regardless of the cell’s other candidates.

  • Fix a digit, e.g. 4.
  • In a chosen box, find every cell where 4 is still legal.
  • If only one cell qualifies, 4 goes there.
A hidden single in motion: each placed 5 blocks part of the box until one cell remains.

Box scanning

The fastest way to find hidden singles is "cross-hatching": for a digit, draw imaginary lines through the rows and columns that already contain it. Those lines eliminate cells in neighbouring boxes; often only one cell in a box is left.

Worked example

  1. Look for the digit 7 in the centre box.
  2. Two rows passing through the box already contain a 7, eliminating six cells.
  3. A column through the box contains a 7 as well, eliminating two more.
  4. Only one cell in the box can still hold 7.
  5. Place 7 there — a hidden single.

Try it yourself

6
8
9
3
2
4
2
5
8
3
5
6
2
8
3
9
3
4
2
8
4
3
1
7
4
6
8
9
4
2
9
6
3
7
5
9
5
8
3
7
2

Tap a cell, then a number, to practise.

Frequently asked questions

Why is it called hidden?
Because the winning cell often shows several pencil marks, so the forced placement is concealed among them. The deduction comes from the unit, not the cell.
Which should I look for first, naked or hidden singles?
Hidden singles via box scanning usually fill the grid fastest early on; switch to naked singles once you have full pencil marks.

Related techniques

Practice: Hidden Single

Put the Hidden Single to work on a live board — free puzzles with notes, hints and four difficulty levels.

Try it on a live board

We use Google Analytics to understand how people use the site. No tracking cookies are set unless you accept. Read our analytics cookie policy.