Hidden Singles Technique

Hidden singles occur when a number can only be placed in one specific cell within a row, column, or 3×3 box, even though that cell may have multiple pencil mark candidates.

Unlike naked singles (which only have one possibility), hidden singles have multiple possibilities in the cell, but one number (the "hidden single") only appears once in that cell's entire row, column, or box. This technique requires careful observation of pencil marks.