A state where a specific digit can only be placed in one cell within a row, column, or block.

A Hidden Single occurs when a specific digit has only one possible placement within a unit (row, column, or block). The cell itself may have multiple candidates, but the logic "this digit can only go here" confirms the placement.

Difference from Naked Single

Naked Single narrows candidates from the "cell's perspective," while Hidden Single narrows placement from the "digit's perspective." This shift in viewpoint makes it harder for beginners to spot. However, once you get the hang of it, you can find them as quickly as Naked Singles.

Block Scanning Method

Focus on a specific digit and examine blocks where it has not yet been placed. Use row and column constraints to narrow down possible cells. If only one cell remains, a Hidden Single is confirmed. Scanning all blocks for each digit from 1-9 is the systematic approach.