The Hidden Singles Technique in Sudoku

Hidden Singles

A Hidden Single emerges in cases where a digit is uniquely allowed to occupy one position on a certain house but that cell accommodates more than

one candidate and the required digit is contextually masked.

Example 1:

4, 6 and 9 could serve as filling values in R3C4 however looking deeper we find that R3C1 and R3C2 have 6 in R2C3 therefore they are useless and in

R3C6 6C6 are blocked therefore leave only the ground element 6 which has space on R3 so it is bound to avail.

Sudoku rule Hidden Singles

Example 2:

In R6C4, the digit 3 is a Hidden Single:

Row 6 (R6): No other cell in R6 can hold 3.
Column 4 (C4) and Block 5 (B5): 3 is excluded elsewhere. Therefore, R6C4 must be 3.

Sudoku rule Hidden Singles

This example shows how identifying a hidden candidate can solve a cell — for related single-candidate strategies, see Full House and Naked Single.