How to play Sudoku

Learn the rules and four logical techniques used by PuzzleMori.

The rules

Fill a 9 × 9 grid with 1–9. Each digit appears once per row, column and 3 × 3 box. Given numbers stay fixed. Arithmetic is not involved; placement and elimination are the task.

Single candidate and hidden single

A single candidate is the only legal number in one cell. A hidden single is the only place for a particular number in a group, even if that cell has other candidates. Scan both cells and whole groups.

Locked candidates and naked pairs

When every candidate for one digit in a box lies on one row, eliminate that digit from the rest of the row. If exactly two cells in a group contain the same two candidates, eliminate both from the other cells. Our hints explain the affected cells before applying an elimination.

Our difficulty labels

Easy uses singles. Medium may need locked candidates. Hard may need pairs as well. If a requested practice level cannot be produced within the generation limit, a solvable easier puzzle is returned and its verified difficulty is shown. An old board needing unsupported techniques is explicitly labelled advanced.