What is Hidden Single (only one place for it)? How to Solve Sudoku

What it is

If a digit fits in only one cell of a row, column, or box, that cell is the answer.

Example: look for where 7 can go in a box. Every other cell already has a 7 in its row or column, so only one cell is left. That cell is 7.

How to spot it

Pick one digit and check the boxes one by one. From each copy of that digit already on the board, trace its row and column to cross out cells. The one cell left is your answer.

Example

Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.

In column 2, the only cell that can hold a 5 is R9C2. Every other empty cell shares a row, column, or box with the 5 at R1C3, R6C9, and R8C7, so it can't be a 5.

→ R9C2 is 5.