Sudoku techniques: learn all techniques from easy to brutal

The easiest sudokus are solved with two techniques that both build on the same rule: each digit can only appear once in each row, column and box. If you learn these two, you can solve all puzzles on Easy level.

At Medium level it is no longer enough to place digits directly. Now it is about removing candidates: when you can prove that a digit cannot go in a cell, the rest of the puzzle becomes easier. Write candidate notes, because that is the key to all techniques from here on.

At Hard level you must see multiple units in context: digits that form rectangles across multiple rows, chains of linked cells and boxes that lock each other. The techniques still only remove candidates, but these are exactly the eliminations that open up the puzzle.

Puzzles at Extreme level require techniques that combine three or more cells in logical reasoning of the if-then type. Try your way through the examples below, step by step, using the same tools the solver uses on your own puzzle.

The hardest puzzles require techniques that follow long logical chains through the entire puzzle. They are in practice proof by contradiction: assume something, follow the consequences and see what does not hold. Try your way through the examples below, step by step, using the same tools the solver uses on your own puzzle.