Hidden Single
Hidden Single is one of the most frequently used techniques in Number Place. By focusing on a specific number, you find cases where it can only be placed in one cell within a row, column, or block. It’s said that this technique alone can fill the majority of cells in easy to intermediate puzzles.
Overview of Hidden Single
A Hidden Single occurs when a particular number can only be placed in one cell within a row, column, or 3×3 block. While Naked Single focuses on ‘a cell,’ Hidden Single focuses on ‘a number.’ The target cell may have other candidate numbers, but since the number in question can only go there, it’s confirmed.
Example
Suppose the top-left 3×3 block has three empty cells, and ‘7’ hasn’t been placed in this block yet. Focus on ‘7’ and check each empty cell: the first and second cells can’t hold ‘7’ because each of their rows already contains ‘7’ in another block. That leaves only the third cell — so ‘7’ is confirmed there.
Tips for Finding Hidden Singles
Check numbers that appear most frequently on the board first — they’re more efficient. For example, if ‘3’ already appears 7 times, you only need to figure out where the remaining 2 go. Checking numbers from ‘1’ through ‘9’ in order is also effective. After one complete cycle, new hints emerge, making the second pass much easier.