Sudoku techniques: beginner to advanced

The most basic technique. If an empty cell has exactly one possible candidate — because the other eight numbers already appear in its row, column, or 3×3 box — that number has to go there. No need to compare with other cells: just look at that one cell.

The reverse of Naked Single: instead of looking at a cell and seeing which candidates are left, you look at a number and see where it can go within a row, column, or box. If that number only fits in one cell of that unit — even if that cell has other candidates pencilled in — it goes there. It's easy to overlook because the cell "looks" like it has several options at first glance.

When, inside a 3×3 box, every cell a specific number could go in is lined up in the same row or column, that number can't appear anywhere else in that row or column outside the box — so it can be eliminated as a candidate from the rest of that row or column, even before you know exactly which cell in the box it will end up in.

When two cells in the same row, column, or box have exactly the same two candidates pencilled in (and only those two), those two numbers are "reserved" for those two cells. They can be eliminated as candidates from every other cell in that same row, column, or box, even before you know which of the two cells gets which number.

The "hidden" version of Naked Pair: two numbers that can only go in the same two cells of a row, column, or box, even if those two cells have other candidates pencilled in besides those two. Once you spot the pair, you can eliminate the rest of the candidates from those two cells — they're left with only those two numbers.

Easy Sudoku solves with just Naked Single and Hidden Single. Medium Sudoku adds Locked Candidates and Naked Pair. Hard Sudoku requires combining several of these techniques, Hidden Pair included, in the same game.