Sudoku Sudoku has a beautiful learning curve. At first you look for obvious missing numbers. Then you start writing candidates. Then a puzzle refuses to move until you spot a pair, a locked candidate, a fish pattern, a wing, a chain, or a uniqueness trick hiding in plain sight.
This is a broad, practical list of Sudoku techniques. It is not meant to replace full tutorials; it is a map. Each entry gives you the name, the purpose, and a short explanation so you can recognise the strategy and decide what to study next. When we already have a full guide, the technique name links to it.
The most useful Sudoku techniques are scanning, cross-hatching, naked singles, hidden singles, locked candidates, naked pairs, hidden pairs, X-Wing, Swordfish, XY-Wing, Simple Colouring, Unique Rectangles, chains, and Almost Locked Sets. Learn them in that order and you will have a strong toolkit for easy, medium, hard, and many expert puzzles.
Best Order to Learn Sudoku Techniques
If you are not sure where to start, use this order. It moves from common, high-value strategies to rarer expert patterns.
| Stage | Techniques to Learn | Why It Matters |
|---|---|---|
| Beginner | Scanning, cross-hatching, full houses, singles | Solves easy puzzles and builds grid awareness. |
| Lower intermediate | Locked candidates, naked pairs, hidden pairs | Starts removing candidates instead of only placing digits. |
| Upper intermediate | Triples, quads, X-Wing, Swordfish | Handles harder grids with repeated candidate patterns. |
| Advanced | Wings, skyscrapers, remote pairs, coloring | Uses relationships between distant cells. |
| Expert | AIC, ALS, uniqueness, Sue de Coq, Medusa | Unlocks puzzles designed for advanced solvers. |
Beginner Sudoku Techniques
These Sudoku techniques are the foundation. Even expert solvers use them constantly because they are fast and reliable.
Scanning
Look across rows, columns, and boxes to see where a digit can still fit. Scanning is the first habit every solver needs because it finds obvious placements without heavy notation.
Cross-Hatching
Choose one digit, then use existing copies of that digit to rule out rows and columns inside a box. If only one cell remains in the box, place the digit there.
Counting
Check a row, column, or box that is nearly complete and count which digits are missing. This is simple, but it prevents many avoidable candidate mistakes.
Full House
A row, column, or box has eight solved digits and one empty cell. The missing digit must go in that final cell.
Last Free Cell
A house has only one open square left. It is similar to a full house, but the emphasis is on the only remaining cell rather than the only missing digit.
Last Possible Number
A cell can accept only one digit because the other digits are already present in its row, column, or box.
Last Remaining Cell
A digit can go in only one cell within a row, column, or box after the other positions have been ruled out.
Candidate Elimination
Remove impossible digits from empty cells by checking their row, column, and box. Most later Sudoku strategies are just more sophisticated ways to eliminate candidates.
Pencil Marks
Write small candidate numbers inside empty cells. Pencil marks make intermediate and advanced Sudoku techniques possible because they reveal patterns.
Snyder Notation
Mark only pairs of possible positions for a digit inside each box, usually in the corners of cells. This keeps beginner grids cleaner than full candidate notation.
Singles, Pairs, Triples and Quads
Subset techniques use small groups of cells or candidates inside one house. They are among the highest-value Sudoku techniques because they appear often.
Naked Single
A cell has exactly one candidate left. That candidate must be the answer for the cell.
Hidden Single
A digit appears as a candidate in only one cell within a row, column, or box. The cell may contain other candidates, but that digit has only one possible home.
Naked Pair
Two cells in the same house contain the same two candidates and no others. Those two digits can be removed from every other cell in that house.
Hidden Pair
Two digits appear only in the same two cells of a house. Remove all other candidates from those two cells, leaving the hidden pair exposed.
Naked Triple
Three cells in one house contain only three shared candidates between them. Those three candidates can be removed from the rest of the house.
Hidden Triple
Three digits are restricted to the same three cells in a house. Remove extra candidates from those cells to reveal the triple.
Naked Quad
Four cells in one house contain only four total candidates. Those four candidates are locked into those cells and can be eliminated elsewhere in the house.
Hidden Quad
Four digits appear only in four cells within a house. It is powerful but harder to spot than pairs or triples because the useful digits can be buried among extras.
Locked Candidate Techniques
Locked candidates connect boxes with rows or columns. They are often the first major step beyond singles and pairs.
Locked Candidates
A candidate is restricted by the interaction between a box and a line. The two common forms are pointing and claiming.
Pointing Pair
Inside a box, a candidate appears only in two cells on the same row or column. Remove that candidate from the rest of the same row or column outside the box.
Pointing Triple
The same idea as a pointing pair, but the candidate is limited to three cells in one line inside a box.
Box/Line Reduction
Also called claiming. If every possible position for a digit in a row or column falls inside one box, remove that digit from the other cells in the box.
Claiming Pair or Triple
A row or column claims a candidate inside one box. The candidate cannot appear elsewhere in that box.
Fish Techniques
Fish techniques look at one candidate digit across multiple rows and columns. They create a grid of possible positions that forces eliminations.
X-Wing
A candidate appears in exactly two positions in each of two rows, and those positions line up in the same two columns. Remove that candidate from other cells in those columns.
Swordfish
A three-row, three-column version of X-Wing. If a candidate is locked into the same three columns across three rows, eliminate it from the other cells in those columns.
Jellyfish
A four-row, four-column fish pattern. It is rarer than X-Wing and Swordfish, but the same elimination logic applies.
Finned X-Wing
An X-Wing with an extra candidate, called a fin, in one box. Eliminations are limited to cells that see both the fin and the fish line.
Finned Swordfish
A Swordfish with one or more fin cells. It behaves like a near-Swordfish and gives narrower but still useful eliminations.
Finned Jellyfish
A Jellyfish pattern with fin cells. It is uncommon in hand solving, but it follows the same restricted-elimination idea as other finned fish.
Sashimi X-Wing
A broken X-Wing where one corner is missing and a fin keeps the logic alive. It removes candidates only in cells affected by the fin.
Sashimi Swordfish
A Swordfish variant with a missing base candidate and fin support. It is a specialised fish pattern for very hard grids.
Turbot Fish
A single-digit chain with two strong links. It is often explained alongside Skyscraper, Two-String Kite, and Empty Rectangle.
Two-String Kite
Two conjugate pairs for the same digit, one in a row and one in a column, connected through a box. A cell that sees both far ends can lose that digit.
Empty Rectangle
A box pattern where one candidate forms a rectangle-like elimination with a conjugate pair in a row or column.
Skyscraper
Two strong links for the same digit share a base line but have different top endpoints. Any cell seeing both tops can lose that candidate.
Wing Techniques
Wing techniques use a small set of cells that force one of two outcomes. They are easier to use once you are comfortable with bivalue cells.
XY-Wing
Three bivalue cells form a pivot and two pincers. Any cell that sees both pincers can lose the shared pincer candidate.
XYZ-Wing
A three-candidate pivot works with two related wing cells. Cells that see all relevant parts of the pattern can lose the shared candidate.
W-Wing
Two matching bivalue cells are linked by a strong link on one digit. The other digit can often be eliminated from cells that see both bivalue cells.
WXYZ-Wing
A larger wing pattern using four cells and four candidates. It is related to ALS logic and usually appears in expert puzzles.
M-Wing
A short chain pattern built from bivalue cells and strong links. It often acts like a compact XY-Chain.
S-Wing
A less common wing pattern that combines bivalue relationships and strong links to force an elimination at the endpoints.
XY-Chain
A chain of bivalue cells where each adjacent pair shares a candidate. If the chain starts and ends with the same candidate, that candidate can be removed from cells seeing both ends.
Chains and Coloring Techniques
Chains track what must happen if one candidate is true or false. Coloring is a visual way to follow those relationships.
Simple Colouring
Choose one digit and color alternating candidates along strong links. Contradictions or shared visibility can produce eliminations.
Multi-Colouring
Use more than one color chain for the same digit, then compare how the chains interact. It can find eliminations that simple coloring misses.
Remote Pairs
A chain of bivalue cells all sharing the same two candidates. A cell that sees both ends of the chain can lose both candidates.
X-Chain
A chain using one candidate digit and alternating strong and weak links. It proves that at least one endpoint must be true.
Alternating Inference Chain (AIC)
A general chain made from alternating strong and weak inferences. AICs are one of the most flexible expert Sudoku techniques.
Nice Loop
A closed chain of inferences. When the loop returns to its start cleanly, it can prove eliminations or placements around the loop.
Grouped AIC
An AIC where a link can involve a group of candidate positions instead of a single cell. Grouped links make chains more powerful but harder to read.
Forcing Chain
Follow the consequences of a candidate being true or false. If both branches produce the same result, that result is logically forced.
Nishio
Temporarily assume a candidate is true and test whether it leads to a contradiction. Many solvers consider it logical trial rather than a pattern technique.
3D Medusa
A multi-digit coloring system using strong links and bivalue cells. It can find contradictions, forced placements, and eliminations across several digits at once.
Uniqueness Techniques
Uniqueness techniques assume a properly made Sudoku puzzle has one solution. They are popular in expert solving, but some purists prefer not to rely on them.
Unique Rectangle
Four cells form a rectangle that could create two solutions if only two digits remain. Extra candidates in the rectangle can often be eliminated.
Unique Rectangle Type 1
Three rectangle cells contain only the deadly pair and the fourth has extra candidates. Remove the deadly-pair candidates from the fourth cell.
Unique Rectangle Type 2
Two roof cells share one extra candidate. If the deadly pair would otherwise repeat, the shared extra candidate creates eliminations nearby.
Unique Rectangle Type 3
A unique rectangle combines with subset logic. The roof cells and nearby candidates form a naked or hidden subset.
Unique Rectangle Type 4
Strong links inside the rectangle prevent one deadly-pair digit from occupying both roof cells, allowing eliminations of the other digit.
Hidden Rectangle
A rectangle pattern where one or more corners are not obvious at first because the deadly pair is hidden among other candidates.
Avoidable Rectangle
A uniqueness pattern involving solved digits and candidates. It prevents a second solution by avoiding an interchangeable rectangle.
Unique Loop
A larger loop of cells that could create multiple solutions if all cells were reduced to the same two candidates.
BUG and BUG+1
BUG means Bivalue Universal Grave, a near-deadly grid where every unsolved cell has two candidates. In BUG+1, one cell has three candidates and usually contains the key placement.
Almost Locked Sets and Expert Techniques
These Sudoku techniques are less common in casual solving, but they are essential vocabulary for very hard puzzles and advanced solver discussions.
Almost Locked Set (ALS)
An ALS is a group of N cells containing N+1 candidates. If one candidate is removed, the set becomes locked, which creates useful restrictions.
ALS-XZ
Two Almost Locked Sets share a restricted common candidate. A second shared candidate can often be eliminated from cells that see both sets.
ALS-XY-Wing
An ALS-based version of wing logic. Several Almost Locked Sets interact through restricted candidates to force an elimination.
Sue de Coq
A two-sector disjoint subset at the intersection of a box and a line. It uses ALS-style reasoning to remove candidates from both sectors.
Death Blossom
A stem cell interacts with several Almost Locked Sets. No matter which value the stem takes, certain candidates become impossible.
Exocet
An advanced pattern involving base cells, target cells, and digit transfer logic. It is mainly seen in very hard human-solver puzzles.
Pattern Overlay Method
Consider all possible completed patterns for a single digit, then eliminate candidates that appear in no valid pattern. It is more systematic than most hand techniques.
Templates
A template is one possible placement pattern for a digit across the whole grid. Comparing templates can reveal eliminations.
Last-Resort and Solver Techniques
These methods are useful to know, but they are not always considered elegant human Sudoku techniques.
Bifurcation
Split the solve into two branches by assuming one candidate is true or false. It can work, but it is usually treated as trial and error unless the branches are tightly controlled.
Trial and Error
Guess a value and continue solving until it works or fails. It is effective as a last resort but less satisfying than logical techniques.
Backtracking
A computer-solver method that tries values recursively and backs up when a contradiction appears. It is excellent for verification, but it is not how most humans want to solve.
Brute Force
Systematically test possibilities until the solution is found. Brute force proves a puzzle can be solved, but it does not explain the human logic.
Do not try to memorise every Sudoku technique at once. Learn one level, practise until it feels natural, then add the next. For most players, singles, locked candidates, pairs, triples, X-Wing, XY-Wing, and simple coloring cover an enormous amount of real solving.
The strongest Sudoku solvers do not use every technique on every puzzle. They move through a reliable order: place what is obvious, clean up candidates, scan for common subsets, look for box-line interactions, then escalate to fish, wings, chains, uniqueness, and ALS only when the grid demands it.
Frequently Asked Questions
The most important Sudoku techniques are scanning, cross-hatching, naked singles, hidden singles, naked pairs, hidden pairs, locked candidates, pointing pairs, box-line reduction, X-Wing, Swordfish, XY-Wing, Simple Colouring, and Unique Rectangles.
Beginners should learn scanning, cross-hatching, full houses, naked singles, hidden singles, and basic candidate elimination first. These techniques solve many easy and medium Sudoku puzzles without advanced pattern hunting.
A practical order is singles, locked candidates, naked and hidden subsets, basic fish patterns, wing patterns, coloring, chains, uniqueness patterns, and Almost Locked Sets.
No. Advanced Sudoku techniques such as X-Wing, XY-Wing, AIC, ALS-XZ, and Unique Rectangles are logical elimination methods. They may feel complicated, but each one removes candidates using a provable rule.
There are dozens of named Sudoku techniques. The exact count depends on whether variants, subtypes, grouped chains, finned fish, sashimi fish, ALS patterns, and uniqueness subtypes are counted separately.