Sudoku solving techniques
- Naked singles — Naked singles in Sudoku: a cell with only one possible digit left. See the pattern on a real puzzle, why placing it is safe, and the mistakes to avoid.
- Hidden singles — Hidden singles in Sudoku: a digit that fits only one cell of a row, column or box. A real-puzzle diagram, how to scan for it, and common mistakes to avoid.
- Naked pairs — Naked pairs in Sudoku: two cells in a house sharing the same two candidates. See the pattern on a real puzzle, the elimination it allows and common slips.
- Hidden pairs — Hidden pairs in Sudoku: two digits that fit only the same two cells of a house. A real-puzzle diagram, how to spot one, and the candidates it lets you remove.
- Naked triples — Naked triples in Sudoku: three cells whose candidates total only three digits. A real-puzzle diagram, why the other cells lose them, and common slips.
- Pointing pairs and triples — Pointing pairs and triples in Sudoku: a digit in a box confined to one line clears that line. A real-puzzle diagram and the common mistakes to watch for.
- Box/line reduction — Box/line reduction in Sudoku: a digit in a row or column confined to one box clears the rest of that box. A real-puzzle diagram and common mistakes to avoid.
- X-Wing — X-Wing in Sudoku: a digit with two possible cells in each of two rows, lined up in two columns. A real-puzzle diagram, the logic and common mistakes to avoid.
- Swordfish — Swordfish in Sudoku: the X-Wing idea across three rows and three columns. A real-puzzle diagram, why it works, and the cases where the pattern is not valid.
- XY-Wing — XY-Wing in Sudoku: three two-candidate cells, a pivot and two wings, force a shared digit. A real-puzzle diagram, the logic and common mistakes to avoid.
- Simple coloring — Simple coloring in Sudoku: follow a chain of either-or links for one digit and use two colors to find eliminations. A real-puzzle diagram and the pitfalls.
- Unique rectangles — Unique rectangles in Sudoku: avoid a pattern that would give two solutions. A real-puzzle diagram, when the shortcut is safe, and why it needs uniqueness.
Sudoku solving techniques are named patterns that let you place a digit or cross out a candidate without guessing. This series teaches twelve of them in the order a solver usually meets them, from naked and hidden singles up to unique rectangles.
Each lesson has a diagram drawn from a real position by this site's own solver, and each says which difficulty tier of this site first needs it.
The learning path
Work through the lessons in order. Each technique builds on the one before it, and most of them are a larger or more indirect version of an idea you have already seen.
- Naked singles and hidden singles are the foundation. They need no notes and finish every Easy puzzle on this site.
- Naked pairs and hidden pairs introduce sets: two cells that between them use up two digits.
- Naked triples extend sets to three cells, and are first needed in Hard puzzles.
- Pointing pairs and box/line reduction link a box with a row or column. They are first needed in Medium puzzles, earlier than triples, but they work across two groups instead of inside one, so this path teaches them once the set idea is familiar.
- X-Wing and Swordfish are the fish patterns, which track one digit across several rows and columns.
- XY-Wing and simple coloring use forced either-or choices, the first with three cells and the second with chains.
- Unique rectangles use the promise that a puzzle has only one solution, and are what separates this site's Evil tier from the rest.
Which tier needs which technique
This site grades its puzzles by the techniques a solver needs, not by the number of clues. Easy needs naked and hidden singles only. Medium adds pairs, pointing pairs and box/line reduction. Hard adds naked triples and X-Wing. Expert adds Swordfish, XY-Wing and simple coloring. Evil needs unique rectangles or something beyond the techniques implemented here. This is the site's own grading, not an industry standard, and the page on how Sudoku difficulty is graded explains its method and limits.
The technique frequency guide counts how often each technique was used when the site's solver worked through 1,000 stored classic puzzles per tier, cheapest technique first. The numbers on each lesson come from that same committed script.
How the lessons are made
Every diagram is a real position from a stored puzzle, solved up to the step shown by the same engine that powers the in-game hints. The caption under each diagram is the engine's own explanation of that step, so the picture and the hint you would see in the game cannot disagree. The hints work from the digits you have placed rather than from your pencil notes.
How to practise
Play a puzzle at the tier where a technique first appears, with notes on, and use the hint button in escalating steps: the first level shows where to look, the second names the technique, and the third shows the move before you apply it. The main board has all five tiers, and the full solving guide shows how the techniques fit together in a real solve.