What it is
Pick a unit — a row, a column or a box — and pick a digit that is missing from it. Check each empty cell in the unit: can that digit go there? If exactly one cell can take it, it goes there, no matter how many other candidates that cell has.
The word *hidden* is the point. The cell may have four or five possible digits, so it looks unremarkable. What settles it is that nowhere else in the unit can take the digit.
Every other empty cell in this unit is blocked from taking the digit by the same digit in a crossing row or column, so the highlighted cell is the only home left.
How to scan for them
Work one digit at a time across a band of three boxes:
- Choose a digit that already appears six or seven times on the board.
- Find a box in that band that is still missing it.
- Strike out every row and column through the box that already contains the digit.
- If one cell in the box survives, place it — then repeat for the next box in the band.
This is often called cross-hatching, and it is the technique that carries an easy or medium puzzle from start to finish.
Why it beats writing candidates
Cross-hatching needs no pencil marks at all: you are reading the digits already on the board. On easy and medium grids that makes it several times faster than filling in candidate lists and then reading them back.
When it stops working
On a hard grid you will reach a position where no digit has a single home in any unit. That is the point to start writing candidates and to look for pointing pairs and naked pairs.
Common questions
What is a hidden single?
A digit that can only go in one cell of a row, column or box, even though that cell may have several candidates of its own.
What is cross-hatching?
The scanning method used to find hidden singles: strike out the rows and columns that already contain a digit and see which cells in a box survive.
Why is it called hidden?
Because the cell it identifies does not look special — it may have many candidates. What is hidden is that the digit has nowhere else to go.
Which is more useful, hidden or naked singles?
Hidden singles, by a wide margin, because they need no pencil marks. Most solvers scan for them first and only write candidates when scanning dries up.