A hidden single flips the naked single around. Instead of asking “what can this cell be?”, ask “where can this digit go?”. If a digit has exactly one possible cell left in a row, column, or box, it must go there — even when that cell still shows several other pencil marks. The single is “hidden” behind them.
To spot one, pick a digit and sweep a house: cross off every cell that already sees that digit in its own row, column, or box. If only one cell survives the sweep, you have found it.
It works because a house must contain every digit exactly once. The cell may have looked open to many values, but the house has no other home for this one — so the other pencil marks in that cell are just noise.
On the board
In row 2, the cell at column 7 still shows five candidates: 1, 2, 4, 7, and 8. But sweep the 8s. Every other empty cell in row 2 is blocked: column 2 sees the 8 down at R8C2, column 3 the 8 at R1C3, column 4 the 8 at R3C4, columns 5 and 6 share the top-middle box with that same 8, column 8 sees the 8 at R9C8, and column 9 the 8 at R6C9. Row 2 needs an 8 somewhere, and R2C7 is the only cell left that can take it — hidden behind four other pencil marks.
This is the exact board the app's interactive Hidden Single demo uses — step 20 of its 56-step walkthrough, drawn here with its live pencil marks.
