Sudoku techniques: learn all techniques from easy to brutal

The easiest Sudoku puzzles can be solved with two techniques based on the same rule: each digit can appear only once in each row, column and box. Learn both techniques, and you can solve every puzzle at the Easy level.

At the Medium level, placing digits directly is no longer enough. The focus shifts to eliminating candidates: when you can prove that a digit cannot go in a cell, the rest of the puzzle becomes easier. Add candidate notes, as they are essential to every technique from this point on.

At the Hard level, you must consider several units together: digits that form rectangles across multiple rows, chains of linked cells and boxes that lock each other. These techniques still eliminate only candidates, but those eliminations are exactly what opens up the puzzle.

Puzzles at the Extreme level require techniques that combine three or more cells in if-then chains of logic. Work through the examples below step by step, using the same tools that the solver applies to your own puzzle.

The hardest puzzles require techniques that follow long logical chains across the entire puzzle. In practice, they work like proofs by contradiction: make an assumption, follow its consequences and identify what cannot hold. Work through the examples below step by step, using the same tools that the solver applies to your own puzzle.