The extra techniques used by Medium and Hard

This page is a short reference. If you are new to Sudoku, start with the step-by-step how-to guide.

Medium

Locked candidates

If every possible position for one digit inside a 3 × 3 box lies in the same row or column, remove that digit from the rest of that row or column. The reverse pattern—one row or column confined to a single box—works too.

Naked and hidden pairs

A naked pair is two cells in one unit whose candidates are the same two digits; those digits can be removed from the other cells in the unit. A hidden pair is two digits that can occur only in the same two cells; other candidates can be removed from those cells.

Hard

Naked triples

In one row, column, or box, choose three unsolved cells whose combined candidates contain exactly three digits. Those three digits must occupy the chosen cells, so remove them from every other unsolved cell in that unit.

Row and column X-Wings

For one digit, find two rows where that digit has exactly two candidate positions in each row, aligned in the same two columns. The digit must occupy opposite corners of that rectangle, so remove it from those two columns outside the selected rows. The column-based form is symmetric: use two columns with exactly two aligned candidate rows, then remove the digit from those rows outside the selected columns.

How website hints work

A hint recalculates candidates from the current board, ignores pencil notes, explains the logical steps, and offers one placement. Applying it changes one cell and can be undone. If the board changes first, the hint is discarded and must be recalculated.

For worked examples from singles through X-Wings, read the full how-to guide.