When scanning stops working

Stuck on a medium Sudoku? Stop guessing and find one fact

A harder grid may not have a nearly complete row. Do not choose the digit that looks most likely. Write the digits that can still fit in one empty cell. Each possible digit is called a candidate.

One visible proof

When one cell has only one candidate left, that digit is forced. This is called a naked single.

1. Choose one empty cellCheck its row, column, and 3×3 box.
2. Remove used digitsCross out every digit already present in those three places.
3. Use the one candidate leftThe outlined cell has only candidate 4, so 4 must go there.

The outlined cell has only candidate 4 after checking its row, column, and 3×3 box.

✓ Every digit except 4 already appears in the cell’s row, column, or 3×3 box. The placement is certain, not a guess.

Try it yourself

Use this reasoning on a fresh grid.

The interactive lesson shows one example, then lets you make the next check.

Practice a forced move free →No account needed. Your progress stays on this device.