Hidden Single (Unique Candidate)

If a cell is the only one in a row, column or block that can take a particular value, then it must have that value. All rows, columns and blocks, must contain each of the digits 1 to 9, so if there is only one cell that can hold a particular value, then it must hold that value.

For example, in the partial Sudoku puzzle below, the marked cell is the only one in block five that can hold a 2, and so it must hold a 2.

Sudoku hidden single example

So why is this technique called hidden single? Simply because if you use a computer assistant (such as SadMan Sudoku) that gives you the full and complete candidate listing for all cells, these cells are the only ones to have a certain digit each, but they're hidden amongst the other candidates for the cell. Contrast this to naked single.