Beginner
Hidden single
A digit has only one home left inside its row, column, or box.
Where naked singles ask "what fits in this cell?", hidden singles flip the question: "where in this row (or column, or box) can this digit go?" If the answer is one cell, that's the placement — regardless of how busy the cell still looks on the candidate side.
Most solvers reach for hidden singles before naked singles because they're faster to find: pick a digit, scan a unit, count the empty cells it could occupy. One cell, done.
When the move applies
Hidden singles drive the early and middle game on easy through medium puzzles. They're especially common after a placement clears out a nearby row, column, or box — sweep that neighborhood digit-by-digit and several often appear at once.
The procedure
- Pick a digit and a unit (row, column, or box).
- Strike out any cell where the digit is already blocked by a peer.
- If exactly one empty cell in the unit remains open to that digit, place it.