Sudoku If you are learning sudoku, the hardest part is not always the logic. Sometimes it is the language. Solvers talk about houses, givens, candidates, pencil marks, locked candidates, X-Wings, ALS, and unique rectangles as if those words are obvious. This glossary makes them obvious.
This guide is designed as a complete reference for Sudoku terms, Sudoku definitions, and common Sudoku terminology. Start with the basics if you are new, or jump straight to the advanced technique glossary if you already solve hard puzzles.
Sudoku terminology is the shared vocabulary used to describe the grid, possible numbers, solving steps, and puzzle types. Learning the terms makes strategy guides easier to follow and helps you explain your own solving logic clearly.
Basic Sudoku Grid Terms
These are the words used to describe the physical structure of a standard 9×9 Sudoku puzzle.
Sudoku
A logic puzzle where you fill a grid so that every row, column, and region contains each required digit exactly once. Standard Sudoku uses a 9×9 grid and the digits 1–9.
Grid
The full Sudoku board. In classic Sudoku the grid has 81 cells arranged in 9 rows and 9 columns.
Cell
One square in the grid. A cell can contain a given digit, a solved digit, or a list of candidates.
Row
A horizontal line of 9 cells. Every row must contain the digits 1–9 once each.
Column
A vertical line of 9 cells. Every column must also contain the digits 1–9 once each.
Box / Block
One of the nine 3×3 regions in a classic Sudoku grid. Many solvers say box, block, or region interchangeably.
Region
Any outlined area that must contain every digit once. In classic Sudoku, regions are 3×3 boxes; in Jigsaw Sudoku, regions have irregular shapes.
House
Any row, column, or box. Standard Sudoku has 27 houses: 9 rows, 9 columns, and 9 boxes.
Unit
Another word for a house. Some guides use unit to mean row, column, or box.
Band
A group of three horizontal boxes, covering three rows. There are three bands in a 9×9 grid.
Stack
A group of three vertical boxes, covering three columns. There are three stacks in a 9×9 grid.
Clue / Given
A digit printed in the puzzle at the start. Givens cannot be changed.
Empty Cell
A cell without a final digit yet. Empty cells are solved by logic, not guessing.
Peer
Any cell that shares a row, column, or box with another cell. Peers cannot contain the same digit.
See / Sees
If two cells share a house, they “see” each other. A candidate can often be removed from cells that see a certain pattern.
Notation, Candidates and Pencil Marks
Notation terms are the bridge between looking at a puzzle and talking about a solve. They are especially important in strategy explanations.
R1C1
Coordinate notation meaning row 1, column 1. R7C4 means the cell in row 7 and column 4.
Candidate
A digit that could still legally fit in an empty cell. Candidates are narrowed down as you solve.
Pencil Mark / Note
A small candidate written inside a cell. Pencil marks let you track possibilities before a digit is confirmed.
Full Candidate Grid
A grid where every empty cell has all currently possible candidates filled in. This is common for harder puzzles.
Elimination
Removing a candidate from a cell because logic proves it cannot be correct.
Placement
Writing a final digit into a cell after logic proves it must go there.
Bivalue Cell
A cell with exactly two candidates left. Bivalue cells are important in wings, chains, and colouring.
Trivalue Cell
A cell with exactly three candidates left. Some advanced patterns use a trivalue pivot.
Strong Link
A relationship where a digit has only two possible positions in a house, so if one is false the other must be true.
Weak Link
A relationship where two candidate positions cannot both be true, though both might be false.
Conjugate Pair
Two candidates for the same digit that form a strong link in a row, column, or box.
Contradiction
A result that breaks a Sudoku rule, such as two 5s in one row or a cell with no candidates. Contradictions prove the assumption was wrong.
Do not try to learn every advanced term at once. Learn cell, house, given, candidate, naked single, and hidden single first. Those six terms unlock most beginner explanations.
Puzzle Quality and Difficulty Terms
These words describe the puzzle itself: whether it is valid, how hard it is, and what kind of logic it requires.
Valid Puzzle
A puzzle that follows Sudoku rules and has at least one solution.
Unique Solution
A puzzle with exactly one solution. Well-made Sudoku puzzles should have a unique solution.
Invalid Puzzle
A puzzle with no solution, multiple solutions, or a direct rule conflict.
Minimal Sudoku
A puzzle where removing any one given would make the puzzle have multiple solutions. Minimal does not always mean extremely hard.
17-Clue Sudoku
A valid classic Sudoku with only 17 givens. For standard 9×9 Sudoku, 17 is the fewest known number of clues possible for a unique puzzle.
Difficulty Rating
A label such as easy, medium, hard, expert, or diabolical. Ratings usually reflect the hardest technique required, not just the number of givens.
Logical Solve
A solve completed by deduction only, without guessing or trial and error.
Guessing / Trial and Error
Testing a candidate without a logical proof. Many solvers avoid guessing, though some advanced methods formalize assumptions carefully.
Backtracking
A search method where a solver tries possibilities and reverses when a contradiction appears. Computers use it often; human strategy guides usually focus on logic.
Solving Technique Glossary
Sudoku techniques are named patterns that let you place digits or eliminate candidates. This section moves roughly from beginner to advanced.
Naked Single
A cell with only one candidate left. That candidate must be the final digit.
Hidden Single
A digit that appears as a candidate in only one cell within a row, column, or box. It is “hidden” because the cell may contain other candidates too.
Last Digit
The only missing digit in a completed row, column, or box.
Naked Pair
Two cells in the same house that contain exactly the same two candidates. Those two digits can be removed from all other cells in that house.
Hidden Pair
Two digits that appear only in the same two cells within a house. Other candidates can be removed from those two cells.
Naked Triple / Quad
Three or four cells in one house whose combined candidates are limited to three or four digits, allowing those digits to be removed elsewhere in the house.
Hidden Triple / Quad
Three or four digits that can only occupy the same three or four cells in a house, letting you remove other candidates from those cells.
Locked Candidates
A box-line interaction where a candidate is restricted to one row or column inside a box, or restricted to one box inside a row or column.
Pointing Pair / Triple
A locked-candidates pattern where all candidates for a digit in one box lie in a single row or column. Remove that digit from the rest of the row or column.
Claiming / Box-Line Reduction
The reverse locked-candidates pattern: a digit in a row or column is confined to one box, so it can be removed from the rest of that box.
X-Wing
A fish pattern where a digit appears in exactly two cells in each of two rows, and those cells share the same two columns. The digit can be removed from other cells in those columns.
Swordfish
A larger fish pattern using three rows and three columns. It works like an X-Wing but across three lines.
Jellyfish
A four-row, four-column fish pattern. It is rarer than X-Wing or Swordfish but uses the same elimination idea.
Finned Fish
A near fish pattern with an extra candidate called a fin. Eliminations are limited to cells that see both the fin and the fish target area.
XY-Wing
A three-cell pattern with one bivalue pivot and two bivalue wings. It eliminates a shared candidate from cells that see both wings.
XYZ-Wing
A wing pattern with a three-candidate pivot and two bivalue wings, eliminating the shared candidate from cells that see all relevant parts.
W-Wing
A chain pattern using two matching bivalue cells connected by a strong link on one candidate.
Skyscraper
A single-digit pattern made from two strong links that share one base direction, allowing eliminations from cells that see both roof candidates.
Remote Pairs
A chain of bivalue cells with the same two candidates. Cells that see both ends of the chain can lose those candidates.
Simple Colouring
A chain technique that colours conjugate pairs for one digit in two alternating colours to find contradictions or eliminations.
Advanced Strategy Terms
Advanced terms often describe families of logic rather than one small pattern. You will see these in expert guides and solver reports.
Chain
A connected sequence of logical links. Chains track what must be true or false if a candidate is chosen.
Forcing Chain
A chain that follows consequences from an assumption until it proves a placement or elimination.
AIC
An Alternating Inference Chain, a chain that alternates strong and weak links to prove an elimination or placement.
ALS
An Almost Locked Set: a group of N cells in one house containing N+1 candidates. ALS logic becomes powerful when two ALS groups interact.
Restricted Common Candidate (RCC)
A candidate shared by two ALS groups in such a way that it cannot appear in both groups at once.
ALS-XZ
An advanced ALS technique where two Almost Locked Sets connected by a restricted common candidate allow eliminations of another shared candidate.
Sue de Coq
A two-sector subset pattern that often uses Almost Locked Sets around a box-line intersection.
3D Medusa
A multi-digit colouring method that follows strong links through cells and candidates, not just one digit.
Unique Rectangle
A uniqueness pattern based on avoiding a deadly rectangle that would create two solutions.
Deadly Pattern
A candidate arrangement that would allow multiple solutions if left unresolved. Uniqueness techniques use this idea.
BUG / BUG+1
BUG means Bivalue Universal Grave, a grid state where every unsolved cell is bivalue. BUG+1 has one extra candidate that can often be resolved by uniqueness logic.
Digit Forcing
Testing the consequences of a digit in a specific cell or house until all branches agree on a result.
Sudoku Variant Terms
Variant terminology describes extra rules added on top of standard Sudoku.
Killer Cage
A dashed or outlined group of cells in Killer Sudoku. The digits in the cage must add to the printed cage total and usually cannot repeat.
Cage Sum
The total that all cells inside a Killer Sudoku cage must reach.
Thermometer / Thermo
A line with a bulb where digits must strictly increase from the bulb to the end.
Arrow
A line-and-circle clue where the digits along the arrow sum to the digit in the circle.
Kropki Dot
A dot between adjacent cells. A white dot means consecutive digits; a black dot usually means one digit is double the other.
Anti-Knight
A rule saying identical digits cannot be a chess knight move apart.
Diagonal Sudoku / X-Sudoku
A variant where the two main diagonals must also contain the digits 1–9 once each.
Jigsaw Sudoku
A variant with irregular regions instead of standard 3×3 boxes.
Samurai Sudoku
A multi-grid puzzle made from five overlapping 9×9 Sudoku grids.
Sandwich Sudoku
A variant where outside clues give the sum of digits placed between 1 and 9 in that row or column.
Skyscraper Sudoku
A variant where outside clues tell how many “buildings” are visible when digits are treated as heights.
Common Sudoku Abbreviations
Solver communities use abbreviations to keep explanations short. Here are the ones you are most likely to see.
| Abbreviation | Meaning | Example |
|---|---|---|
| R | Row | R4 means row 4 |
| C | Column | C8 means column 8 |
| B | Box | B5 means the center 3×3 box |
| R1C1 | Row-column coordinate | Top-left cell |
| PM | Pencil mark | Small candidate note |
| LC | Locked candidates | Pointing or claiming |
| UR | Unique rectangle | A uniqueness pattern |
| ALS | Almost Locked Set | N cells with N+1 candidates |
| AIC | Alternating Inference Chain | Strong and weak links alternating |
| RCC | Restricted Common Candidate | An ALS connector |
When a solving guide uses an unfamiliar word, look it up here, then return to the example. Sudoku terminology is cumulative: once you understand candidates, houses, strong links, and eliminations, most advanced technique names become much less intimidating.
Learning the vocabulary will not solve the puzzle for you, but it gives you the map. The next time a guide says “eliminate 7 from every cell that sees both pincers,” you will know exactly which cells to inspect and why the step works.
Frequently Asked Questions
The most important beginner terms are cell, row, column, box, house, given, candidate, pencil mark, elimination, naked single, and hidden single. Those words explain the grid and the first logic most solvers use.
R1C1 means row 1, column 1. It is a compact coordinate system used to identify any cell in the grid. R5C7 means the cell in row 5 and column 7.
A house is any row, column, or box that must contain each digit once. Standard Sudoku has 27 houses: 9 rows, 9 columns, and 9 boxes.
A clue, also called a given, is a digit printed in the puzzle at the start. A candidate is a possible digit that could still go in an empty cell while you solve.
Advanced Sudoku terms include X-Wing, Swordfish, Jellyfish, XY-Wing, W-Wing, Skyscraper, Remote Pairs, ALS, AIC, Unique Rectangle, BUG+1, and forcing chain.