What is the trick to solving a Sudoku puzzle?

A naked single is a cell where only one digit can possibly go. Scan each empty cell and count how many digits are already placed in its row, column, and 3×3 box. If only one digit is missing from all three, that is your naked single - Place it immediately.

Most Easy and some Medium puzzles can be completed using naked singles alone. Always start here before reaching for anything more complex.

A hidden single occurs when a digit can only go in one cell within a row, column, or box - even though that cell has multiple candidates. Scan each unit asking: where can this digit go? If only one cell allows it, place it.

Hidden singles unlock a large proportion of Medium puzzles. Practising digit-by-digit scans through each box builds this pattern recognition quickly.

Once scanning runs dry, switch to pencil marks (candidate notation). For each empty cell, write every digit that could possibly go there. This makes the remaining logic visible and prevents errors.

With pencil marks in place you can spot patterns that are invisible during raw scanning - Pairs, triples, and pointing sets.

A naked pair is two cells in the same unit that each contain exactly the same two candidates. Because those digits must occupy those two cells, remove them from every other cell in the unit. The same logic extends to naked triples (three cells, three candidates).

Hidden pairs work in reverse: two digits that can only go in the same two cells within a unit, letting you clear all other candidates from those cells.

Harder puzzles require X-Wing (a digit locked to two rows in exactly two columns), Swordfish (extends X-Wing to three rows), and chain techniques. These are rare in Easy and Medium but essential for Hard and above.

The core principle never changes: each digit appears exactly once in every row, column, and box. Apply that constraint in increasingly subtle ways and every puzzle yields.