Hidden Singles

What is a hidden single?

A hidden single is a digit that can fit in only one cell of a row, a column, or a box. The cell itself may still have several candidates, which is why the single is "hidden". But since every unit must contain each digit exactly once, the digit has to go in its only available spot.

Cross-hatching, step by step

1. Choose a digit, for example 3, and a box that does not contain it yet.
2. Draw imaginary lines through every row and column that already contains a 3 and crosses the box.
3. Cross out filled cells. Cells that already hold a digit are not available either.
4. Count what is left. If one cell remains, place the 3 there.

Repeat for every digit and every box, then do the same for rows and columns. This scan is the fastest way to make progress at the start of any puzzle.

Tip

Start with the digits that are already placed most often. If 7 appears six times on the grid, the three missing 7s are usually easy to locate with cross-hatching.