What is a Hidden Single?

A Hidden Single occurs when a specific digit can only be placed in exactly ONE cell within a given row, column, or 3x3 block, even if that cell currently has multiple other candidates.

How to Spot Them

Unlike a "Naked Single" where a cell has only one possible candidate because all other 8 digits see it, a Hidden Single requires you to look at the group (the row, column, or block).

Ask yourself: "Where can the number 7 go in this specific 3x3 box?" If you scan the intersecting rows and columns and realize that every other empty cell in the box is blocked by a 7 elsewhere, then the remaining cell MUST be a 7, no matter what other pencil marks it might have.