Sudoku

Solving techniques

Start by looking for a cell where only one number fits. Use pencil marks to keep track of the possible numbers, called candidates. Each puzzle has one solution.

A naked single is a cell with only one candidate left. Ahidden single is a digit that fits in only one cell of a row, column or box. Those two alone will finish any easy puzzle.

A naked pair is two cells in a unit sharing the same two candidates — those digits cannot go anywhere else in that unit. Pointing pairsand box/line reduction are the two directions of the same idea: when a digit's possible places in a box all sit in one line, or its places in a line all sit in one box, you can eliminate it from the rest.

An X-wing occurs when a digit has just two possible columns in each of two rows, and those columns match. You can remove that digit from the other cells in those columns. The same rule works with rows and columns swapped.