Sudoku techniques guide
This guide explains the logic patterns behind our Easy–Expert daily puzzles. Each tier is generated and analysed so the label matches the hardest technique you actually need, not how many cells are blank. Coaching hints on the play page name these steps as you solve.
Naked single
An empty cell lists only one possible digit after you cross off everything used in its row, column, and 3×3 box. You can place that digit immediately. Most beginners start by scanning for naked singles row by row.
Hidden single
Within one row, column, or box, a digit can appear as a candidate in only one cell — even if that cell still shows other pencil marks. The digit must go there; the other candidates in the cell are distractions. Hidden singles often unlock several naked singles in neighbouring units.
Naked pair
Two cells in the same row, column, or box share exactly the same two candidates (and no others). Those two digits must occupy those two cells, so you can eliminate them from every other cell in that unit. This is a common Medium-tier step when singles dry up.
Pointing pair / pointing triple
When all candidates for a digit inside one 3×3 box lie in a single row or column, that digit cannot appear elsewhere in that row or column outside the box. You eliminate candidates along the line. The reverse idea — a line’s candidates for a digit confined to one box — is called box-line reduction.
X-wing
Suppose digit D appears as a candidate in exactly two cells in row A and exactly two cells in row B, and those four cells form a rectangle (same two columns). Then D must occupy one cell in each of those rows in those columns, so you can remove D from all other cells in those two columns. Hard-tier puzzles on this site often require at least one X-wing-style elimination.
Swordfish and beyond
Swordfish extends the X-wing idea to three rows and three columns. Expert-tier dailies may need chain-style logic beyond swordfish. If you are learning, work through Easy and Medium archives with hints enabled before jumping to Expert.
How this site uses techniques
- Easy — singles only.
- Medium — adds pairs and box-line style steps.
- Hard — typically requires an X-wing (or equivalent).
- Expert — deeper chains and advanced patterns.
Read how to play for site controls, or open the daily puzzle and use Hint to see the next named technique on your board.
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