A first candidate-removal technique

Locked candidates in Sudoku: use a 3×3 box to clear a row

Look at every candidate for one digit inside a 3×3 box. If all of them lie on the same row, the box must place that digit somewhere on that row.

One visible proof

The box reserves the digit on the shared row. This is called a locked candidate. Starting from the box and clearing the row is also called pointing.

1. Follow one candidate in the boxThe top-left box can place 5 only in its first two cells.
2. Notice the shared rowBoth possible 5s are in row 1.
3. Remove 5 outside the boxRow 1 will receive its 5 inside the box, so other cells in row 1 cannot contain 5.

The box can place 5 only on row 1, so row 1 loses candidate 5 outside the box.

✓ Whichever of the two box cells receives 5, row 1 receives its 5 inside that box. Candidate 5 can leave the outlined cells outside the box.

Try it yourself

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

Practice locked candidates free →No account needed. Your progress stays on this device.