Sudoku Pattern Library
A lot of Sudoku guides explain techniques one page at a time. This library is different: it is designed as a quick mental map of the patterns you should recognise while solving. Think of it as the bridge between “I know the term” and “I can actually spot this in a live puzzle.”
If you are building skill, do not try to memorise every advanced idea at once. Learn the pattern families in order, understand the visual trigger for each one, and use this page as a reference when a hard board starts to feel noisy.
How to Use This Pattern Library
- Start with the easiest pattern family you do not yet trust. For most players, that means Singles first, then Locked Candidates, then Pairs.
- Look for the visual trigger, not just the definition. A technique becomes useful when you know what kind of candidate shape should make you suspicious.
- After every elimination, rescan for easier logic. Advanced patterns often create simple singles immediately after they fire.
Level 1: Singles You Should Recognize Instantly
Naked Single: one cell has only one candidate left.
Hidden Single: one digit has only one legal location in a row, column, or box.
These are still the most important patterns in Sudoku. Even expert puzzles often open up because a difficult elimination elsewhere creates an ordinary single. If you miss singles, the rest of the pattern library becomes much harder to use correctly.
Level 2: Locked Candidates
Pointing Pair / Triple: within one 3×3 box, a digit can only appear in one row or one column, so that digit can be removed from the rest of that line outside the box.
Box/Line Reduction: the same idea in reverse; a digit in a row or column is confined to one box, so it can be eliminated from the other cells in that box.
What to look for: a candidate that keeps appearing in a narrow strip through a box. These patterns are often the first real step beyond beginner Sudoku because they train you to think in two intersecting houses at once.
Level 3: Pairs and Triples
Naked Pair / Triple: two or three cells in one house contain only the same two or three digits between them, so those digits can be removed from every other cell in the house.
Hidden Pair: two digits can only go in the same two cells, so all other candidates can be removed from those cells.
What to look for: either a cluster of cells with tiny matching candidate sets, or a pair of digits that keep recurring in exactly the same two positions. These are the patterns that teach you to stop reading cells one by one and start reading candidate structure.
Level 4: Fish Patterns
X-Wing: one digit is restricted to the same two columns across two rows (or the same two rows across two columns).
Swordfish: the same idea extended across three rows and three columns.
What to look for: a single digit that keeps showing up in narrow bands. Fish patterns are not about the whole puzzle; they are about one digit becoming strangely over-organized across multiple houses.
For a full walkthrough, see Advanced Sudoku Techniques.
Level 5: Uniqueness and Chain Patterns
Unique Rectangle: a deadly four-cell ambiguity is avoided because the puzzle is assumed to have one valid solution.
Y-Wing: three bivalue cells form a pivot-and-wings chain that forces a shared candidate to be eliminated elsewhere.
Simple Coloring: strong links on one digit create a two-color chain; contradictions or mutual visibility allow eliminations.
What to look for: repeated bivalue cells, rectangular two-candidate structures, or one digit that has many strong links. These patterns are less common than singles and pairs, but they matter on harder boards because they create clean, high-value eliminations.
Recommended Training Order
- Begin with basic Sudoku logic until Naked Singles and Hidden Singles feel automatic.
- Add intermediate techniques such as Locked Candidates, Naked Pairs, Hidden Pairs, and Triples.
- Study advanced classic patterns such as X-Wing, Swordfish, Unique Rectangle, Y-Wing, and Coloring.
- Then apply the same discipline to Killer Sudoku and X-Sudoku, where the pattern language changes but the same habit of structured scanning still matters.
What Most Players Get Wrong About Patterns
Most hard puzzles still hide simpler logic after every good elimination. If you hunt X-Wings before rescanning for singles, you make the solve harder than it needs to be.
Knowing what a Swordfish is in theory is not the same as noticing when one digit is confined to three suspicious rows and columns. Train the trigger, not just the vocabulary.
Pattern solving depends on accurate candidates. If your notes are out of date, advanced logic turns into guesswork very quickly.
Where to Practice
- Medium Sudoku — good for practicing singles, notes, and first pair patterns
- Evil Sudoku — the best bridge into real advanced logic
- Extreme Sudoku — where fish, chains, and deeper candidate discipline matter most
- Daily Sudoku — useful for variety and consistency
If you are stuck mid-solve, use the site’s teaching flow deliberately: the board, the notes, and the explanation system should help you compare what you saw with what the puzzle was actually offering.