Hidden singles: the only place left
Pick a row and a digit it still needs. If only one cell can take it, it goes there. This is “exactly one” at work.
How it works
A naked single asks “what can this cell be?” A hidden single asks the other question: “where can this digit go?”
Every row must hold each digit exactly once. The “at most one” half rules places out. The “at least one” half is the new tool: the digit has to go somewhere in the row. When every place but one is ruled out, it goes in the last one.
The cell itself may still have several candidates. That is why these are called hidden. You find them by looking at a digit, not at a cell.
Both halves of exactly one
The 2s already on the board use “at most one”: box 1 has its 2, so no other cell in box 1 can be 2. Each of those facts removes a place.
The last step uses “at least one”: row 1 has a 2 somewhere. One place is left, so it is there. Naked singles only need the first half. Hidden singles need both.
How to scan for them
Pick a digit that already appears a lot. For each box that still lacks it, follow that digit’s rows and columns across the box. Often only one cell is left uncovered.
Worth memorizing
A hidden single is a digit with one place left.
In a row, a column or a box.
Scan by digit, not by cell.
Pick a digit, pick a house, look for the one gap.
The cell can have other candidates.
The house decides, not the cell.
Practice
0 of 5 rightWhere does 3 go in row 2?
Where does 5 go in column 1?
Where does 2 go in column 3?
Row 4 needs 4, 5 and 6. Which of them has only one place left?
The ringed cell has four candidates, but one digit has nowhere else to go in row 1. Which?