Hidden Single

Difficulty level: medium

A hidden single flips the naked-single question around. Instead of asking “what fits in this cell?”, ask “where can this number go in this unit?”. If a row, column, or box has exactly one cell that can take a given number, the number must go there — even if that cell could superficially hold other values too.

Hidden singles are the workhorse of medium puzzles. They hide in plain sight because the target cell usually has several pencil-mark candidates; only by tracking the number across the whole unit do you see that every other cell is blocked.

The classic scan is called cross-hatching: pick a number, and sweep each box looking at which of its cells are blocked by that number appearing in intersecting rows and columns.

Worked example

The top-left box needs a 5. Rows 2 and 3 already have 5s, and the other row-1 cells are taken — so the 5 is forced into the highlighted corner.
(highlighted) 3 8
5
5

How to apply it

  1. Pick a number that already appears several times on the board.
  2. For each box missing that number, mark which empty cells could legally take it (no copy in the same row or column).
  3. If only one cell in the box qualifies, place the number there.
  4. Repeat per row and per column — hidden singles live in all three unit types.

Practice it

The fastest way to internalize the hidden single is to use it. Play a free medium puzzle — the in-game hint system points out exactly this pattern when it appears, or browse the full technique library.