Hidden Single
A Hidden Single turns the Naked Single's logic around. Instead of asking "what can this cell be?", you ask "where can this digit go?". If a digit has only one possible home in a row, column or box, it must go there — even when that cell still shows several other candidates. The single is hidden because the cell itself looks undecided; only counting the digit's options across the whole unit reveals the placement.
The classic way to spot them is cross-hatching. Pick a digit and a box, and trace the rows and columns that already contain that digit through the box. Suppose you are placing 7 in the top-left box: row 1 already has a 7 elsewhere, row 2 has a 7 elsewhere, and column 3 has one too. Every cell of the box is eliminated except r3c1 — so r3c1 is 7, regardless of what other pencil marks it carries.
The same logic works along a line. If the 4s in row 6 are blocked from every cell except r6c8 — by 4s standing in the other columns and boxes — then r6c8 is 4. Note the contrast with a Naked Single: there the cell has one option; here the digit has one home.
Hidden Singles are the workhorse of easy and medium puzzles, and most solvers hunt them digit by digit before anything else. Whenever a digit already appears seven or eight times on the grid, its remaining placements are usually Hidden Singles waiting to be read off.
- Cross-hatch digits that appear often on the grid first — the more copies placed, the more cells are blocked.
- Check all three unit types: a digit can be hidden in a box, a row or a column independently.
- Don't trust the cell's pencil marks alone — a Hidden Single sits in a cell that still shows other candidates.