TECHNIQUES · ADVANCED

Advanced Sudoku techniques

Hard Sudoku is not about guessing. It is a chain of candidate eliminations, from singles to box-line interactions and pairs.

Hidden single

If a digit can appear in only one cell inside a row, column, or box, that cell is forced even when other candidates are written there.

Locked candidates

When all candidates for a digit inside one box lie on the same row or column, that digit can be removed from the rest of that row or column.

Naked pair

If two cells in one unit contain exactly the same two candidates, those two digits can be removed from every other cell in that unit.

Find cells with exactly two candidates.
Check whether the same pair appears twice in the same row, column, or box.
Remove that pair from the other cells in the unit.

Example: an XY-Wing triangle — among advanced techniques, one of the most reliable ways to break a stalled board.

Practice order

Clean candidatesReduce possible digits accurately before using named techniques.
Check singlesFind naked singles and hidden singles first.
Use box-line logicApply locked candidates to remove candidates.
Look for pairsUse naked or hidden pairs when the board stalls.

Walk through an XY-Wing (advanced example)

Step 1 of 4

XY-Wing walkthrough

As a representative advanced technique, walk through an XY-Wing. The pivot at (row 5, col 5) holds {1, 2}.

Start a hard puzzle Technique tutorial