Sudoku strategies: from naked pairs to X-Wings

Once you've mastered scanning and naked singles (covered in our beginner's guide), the named Sudoku strategies are the next level. Each one is a small rule about candidate elimination — together they unlock every “Hard” and most “Expert” puzzles you'll meet.

Hidden singles

A naked single is a cell where only one candidate remains. A hidden single is the inverse: within a row, column, or box, a digit that can only legally sit in one cell — even if that cell still has other candidates pencilled in.

To find hidden singles, walk each digit one at a time. In every box, ask which cells could hold that digit. If only one cell can, it's a hidden single, regardless of what other candidates the cell carries.

Naked pairs

Two cells in the same row, column, or box, both carrying the same two candidates and nothing else. Those two digits are locked into those two cells — you don't know which order yet, but you can confidently eliminate both candidates from every other cell in the shared unit.

Naked pairs feel small, but they unblock a stuck grid more often than any other strategy. Watch for them whenever you finish a scan and nothing new opens up.

Pointing pairs

When a digit's only candidates inside a 3×3 box all sit in the same row (or column), that digit must end up in that row inside the box. You can't place it yet, but you can eliminate that digit from the rest of the row outside the box. The elimination often triggers a hidden single elsewhere.

Hidden pairs and box-line reduction

Hidden pairs are the inverse of naked pairs: two candidates that only appear in two cells of a unit. The two cells must hold those two candidates and nothing else, so any other candidates in those cells can be erased.

Box-line reduction is the opposite of pointing pairs: when a digit's only candidates within a row sit inside a single 3×3 box, you can eliminate the digit from the rest of the box. Same idea, different direction.

X-Wing

A more advanced technique. Look for a digit that appears as a candidate in exactly two cells of one row, and exactly two cells of another row, with the candidate cells aligned in the same two columns. Those four cells form a rectangle (“X-Wing”) — the digit must end up in two opposite corners of the rectangle, so you can eliminate the digit from the rest of both columns.

X-Wings are rare on Easy/Medium puzzles. They start appearing on Hard, and on Expert grids you sometimes need two of them in the same puzzle.

The order to learn them

1. Hidden singles — the highest-ROI strategy after naked singles.
2. Naked pairs — the most-used elimination technique on Medium.
3. Pointing pairs — unblocks Hard grids.
4. Hidden pairs and box-line reduction — same idea, more reps.
5. X-Wings — Expert puzzles only.

Beyond X-Wings the strategies get exotic — Swordfish, XY-Wings, chains. They're elegant, but you'll meet them once a month at most. Master the five above and you can solve almost every Sudoku we publish.