Sudoku rules

Classic 9 × 9 Sudoku

Rules of Sudoku

Sudoku is a placement puzzle built on elimination. Complete the nine-by-nine grid so each digit from 1 through 9 appears once in every row, column, and outlined box.

The objective: Complete the grid without repeating 1–9 in any row, column, or box.

Treat the given digits as fixed clues.

Starting digits cannot be changed. Each clue removes that digit as a possibility from the rest of its row, column, and three-by-three box.

This 5 rules out every shaded square for any other 5.

Use 1 through 9 once in every row.

A completed horizontal row contains each digit exactly once. Scan for a digit that has only one legal square left in a row.

“Only one digit is absent. Name it before you place it.”

Orion
Eight digits are placed in this row, and 5 is the one missing. It goes in the open square.

Use 1 through 9 once in every column.

Vertical columns follow the same rule. A placement must satisfy its row and column at the same time.

Eight digits are placed in this column, so the last square must be 2.

Use 1 through 9 once in every 3 × 3 box.

The bold outlines divide the board into nine boxes. Each box independently contains all nine digits without repetition.

One of nine boxes
Missing a 4
Each box holds every digit once, separately from the rows and columns.

Use notes, then validate the solution.

Notes record candidates inside a square. Remove candidates as peers are solved. A full grid with no duplicates is checked automatically.

Notes
Answer
Keep both candidates in the square until another digit rules one out.