Singles

The two foundational placements: one candidate left in a cell, or one possible cell for a digit in a row, column or box.

A selected Sudoku cell with only candidate 8 remaining

Naked Singles

A cell with exactly one candidate remaining.

Candidate 8 highlighted as the only possible 8 in a Sudoku box

Hidden Single

The only cell for a digit within a row, column or box.

The two kinds of Sudoku single

Naked Singles and Hidden Singles are the core placement techniques in candidate-based Sudoku solving. A Naked Single is a cell with only one candidate left. A Hidden Single is different: a digit may share its cell with other candidates, but it has only one possible position within a row, column or box. Scanning both cells and houses is the reliable way to spot them.

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