Sudoku Glossary — Complete Dictionary

Every Sudoku term, technique, and concept defined in plain English. Use this as a reference while you solve, or browse alphabetically to deepen your understanding. Click any technique link to read the full strategy guide with examples.

A

Anti-Knight Sudoku: A Sudoku variant where in addition to the standard rules, no two cells separated by a chess knight's move may contain the same digit. Adds significant difficulty and is popular in competitive Sudoku.

B

Backtracking: A systematic guessing method where you place a candidate, follow its consequences, and reverse the placement if a contradiction is reached. Used by computer solvers and as a last resort by humans.
Bi-location: A condition where a candidate appears in exactly two cells within a unit (row, column, or box). Bi-location patterns are the foundation of X-Wing, Swordfish, and other "fish" techniques.
Bi-value Cell: A cell that contains exactly two candidates. Bi-value cells are the building blocks of advanced chain techniques like Y-Wing and XY-Chain.
Bifurcation: A structured form of guessing where you pick a bi-value cell and explore both possibilities. If one branch leads to a contradiction, the other must be correct. Some purists consider bifurcation distinct from full backtracking.
Box: One of the nine 3×3 sub-grids that make up a standard Sudoku puzzle. Also called a "block" or "region." Each box must contain digits 1–9 with no repetition. There are 9 boxes total in a 9×9 puzzle.
Box-Line Reduction: An intermediate technique. When all candidate positions for a digit within a row or column fall inside the same 3×3 box, the digit can be eliminated from the rest of that box. Also called "Claiming."
Bruteforce: A trial-and-error solving method used when logical techniques are exhausted. You guess a digit, follow it through, and backtrack on contradictions. Well-designed puzzles never require it.

C

Candidate: A digit that could legally go in a particular empty cell based on the current state of the puzzle. Candidates are tracked using pencil marks and reduced through elimination techniques.
Cell: A single square in the Sudoku grid. A standard 9×9 puzzle has 81 cells, each holding exactly one digit from 1–9 in the solved state.
Chain: A sequence of linked cells where the value of one cell forces the value of the next. Chains underpin advanced techniques like XY-Chain, X-Chain, and Forcing Chains.
Claiming: Another name for Box-Line Reduction. The row or column "claims" a digit for a specific box, allowing eliminations within that box.
Clue: A digit pre-filled by the puzzle creator. Also called a "given." Easy puzzles have 36–45 clues; expert puzzles have only 22–27. The minimum for a unique solution is 17.
Column: One of the nine vertical lines of 9 cells in a Sudoku grid. Each column must contain digits 1–9 exactly once.
Crosshatching: A scanning technique where you mentally cross out rows and columns to find where a digit can go within a box. One of the first techniques every Sudoku player learns.

D

Difficulty: A rating that reflects which techniques and how much depth of analysis a puzzle requires. Common levels: Easy, Medium, Hard, Expert. Difficulty correlates loosely with the number of clues but more strongly with the techniques needed.
Diagonal Sudoku: A variant where the two main diagonals must also contain digits 1–9 with no repetition, in addition to standard row, column, and box rules.

E

Elimination: The core process of Sudoku solving: removing impossible candidates from a cell based on what already exists in its row, column, and box. Most techniques are forms of structured elimination.
Empty Cell: A cell that has not yet been filled with a digit. The goal of solving is to determine the unique digit for every empty cell.

F

Fish: A family of techniques that use bi-location patterns across rows and columns. X-Wing is a 2-fish, Swordfish is a 3-fish, Jellyfish is a 4-fish, Squirmbag is a 5-fish.
Forcing Chain: An advanced technique where every possible value of a starting cell forces the same conclusion in another cell. That conclusion can then be applied regardless of the starting cell's actual value.

G

Given: A digit pre-filled in the starting puzzle. Same as a "clue." Givens are usually shown in bold or a different color to distinguish them from cells you have solved.
Grid: The 9×9 layout of cells that forms a standard Sudoku puzzle. The grid is divided into 9 rows, 9 columns, and 9 boxes.

H

Hidden Pair: When two digits can only appear in the same two cells within a unit, those two cells must hold those two digits — and all other candidates in those cells can be eliminated.
Hidden Single: When a digit can only appear in one cell within a row, column, or box, even if that cell shows multiple candidates. Place the digit immediately.
Hidden Triple: When three digits can only appear in the same three cells within a unit, those cells must hold those three digits and any other candidates in them can be eliminated.

I

Intersection: The cells shared between two units (typically a row and a box, or a column and a box). Intersection-based techniques include Pointing Pair and Box-Line Reduction.

J

Jellyfish: A 4-fish technique extending Swordfish to four rows and four columns. Extremely rare and difficult to spot manually.
Jigsaw Sudoku: A variant where the 3×3 boxes are replaced by 9 irregularly shaped regions, each still containing 9 cells. Standard row and column rules apply.

K

Killer Sudoku: A variant where cells are grouped into "cages" with a sum total, and digits within a cage cannot repeat. Combines Sudoku logic with Kakuro-style arithmetic.

L

Last Free Cell: When a row, column, or box has only one empty cell left, the missing digit goes there. The simplest possible solving move.
Locked Candidate: A candidate confined to specific cells within a unit such that placing it elsewhere is impossible. Pointing Pairs and Box-Line Reductions are forms of locked candidates.

M

Mini-Sudoku: Smaller variants of Sudoku such as 4×4 (with 2×2 boxes) or 6×6 (with 2×3 boxes). Often used to teach children the rules before moving to standard 9×9.

N

Naked Pair: Two cells in the same unit containing exactly the same two candidates. Those two digits are locked into those two cells, allowing elimination from other cells in the unit.
Naked Single: A cell with only one possible candidate remaining after elimination. The most fundamental Sudoku technique.
Naked Triple: Three cells in the same unit whose combined candidates contain exactly three digits. The three digits are locked to those cells, eliminating them from elsewhere in the unit.
Note: Another word for pencil mark — a small digit written in a cell to track a candidate.

P

Peer: Any cell that shares a row, column, or box with a given cell. Each cell has 20 peers in standard Sudoku. Peers cannot hold the same digit.
Pencil Marks: Small digits written inside a cell to track which numbers could still legally go there. Essential for medium and harder puzzles.
Pivot: The central bi-value cell in a Y-Wing pattern. The pivot shares units with two "pincer" or "wing" cells.
Pointing Pair: When all candidate positions for a digit within a box lie in the same row or column, eliminating the digit from that row or column outside the box.
Pointing Triple: Same idea as a Pointing Pair but with three cells aligned in a row or column within a box.

R

Region: Another word for a box — one of the 9 3×3 sub-grids. In Jigsaw Sudoku, regions are irregularly shaped.
Row: One of the nine horizontal lines of 9 cells. Each row must contain digits 1–9 exactly once.

S

Samurai Sudoku: A variant made of five overlapping 9×9 grids arranged in a cross. The four outer grids share a box with the central grid.
Scanning: The visual process of looking across rows, columns, and boxes to find where a digit can go. Crosshatching is a structured form of scanning.
Single: A digit that can be placed with certainty in exactly one cell. Naked Singles and Hidden Singles are the two basic types.
Squirmbag: A 5-fish technique. Extremely rare and impractical to find manually — typically only relevant to computer solvers.
Subset: A group of N cells in the same unit whose candidates collectively span exactly N digits. Naked Pairs (subset size 2), Naked Triples (3), and Naked Quads (4) are all subsets.
Swordfish: A 3-fish technique. When a digit appears in 2–3 cells across three rows and those cells span exactly three columns, the digit can be eliminated from those columns elsewhere.

U

Unique Solution: Every properly constructed Sudoku has exactly one valid solution reachable through logic alone. Puzzles with multiple solutions are considered broken.
Unit: A collective term for a row, column, or box — any group of 9 cells that must contain digits 1–9 exactly once.

W

Wing: A non-pivot cell in a Y-Wing or other wing technique. Also called a "pincer." Wings share a unit with the pivot and contain one of its candidates.

X

X-Wing: A 2-fish technique. When a digit appears in exactly two cells across two rows that share the same two columns, the digit can be eliminated from those columns elsewhere.
XY-Chain: An advanced chain technique using a sequence of bi-value cells linked by shared digits. Extends the logic of Y-Wing to longer chains.
XY-Wing: Another name for Y-Wing. Three bi-value cells (pivot + two wings) enable indirect eliminations in cells that see both wings.

Y

Y-Wing: An expert-level technique using three bi-value cells: a pivot (AB), and two pincers (AC, BC). Any cell seeing both pincers loses the candidate C.

Frequently Asked Questions

What does "naked" mean in Sudoku terminology?

In Sudoku, "naked" describes a candidate or set of candidates that is immediately visible without complex analysis. A Naked Single is a cell with only one candidate; a Naked Pair is two cells sharing exactly two candidates. The word emphasizes that the pattern is "exposed" rather than hidden behind other candidates.

What is the difference between "hidden" and "naked" in Sudoku?

Naked patterns are visible by looking at a cell's candidates: if a cell has only one candidate, it is a Naked Single. Hidden patterns require looking across cells: if a digit can only go in one cell within a unit, it is a Hidden Single — even if that cell has many other candidates. Both lead to the same placement, but the perspective differs.

What is a "unit" in Sudoku?

A unit is any group of 9 cells that must contain digits 1–9 exactly once. There are three types: rows, columns, and 3×3 boxes. A standard Sudoku has 27 units total (9 rows + 9 columns + 9 boxes). Most solving techniques work within or across units.

What is a "peer" in Sudoku?

A peer of a cell is any other cell that shares its row, column, or box. Each cell has 20 peers in standard Sudoku. The rule "no two peers may hold the same digit" is the foundation of all elimination logic.

What is a "bi-value cell" and why does it matter?

A bi-value cell contains exactly two candidates. These cells are the building blocks of many advanced techniques: Y-Wing uses three bi-value cells, and chain techniques like XY-Chain extend that pattern. Tracking bi-value cells is a fast way to spot opportunities for advanced eliminations.

What is the minimum number of clues a Sudoku can have?

Mathematicians have proven that the minimum number of clues required for a Sudoku to have a unique solution is 17. Puzzles with 16 or fewer clues always have multiple valid solutions. Most expert-level puzzles use 22–27 clues for the right balance of difficulty and solvability.

What does "fish" mean in Sudoku?

Fish techniques are a family of advanced strategies named for their visual patterns. X-Wing is a 2-fish (two rows and two columns), Swordfish is a 3-fish, Jellyfish is a 4-fish, and Squirmbag is a 5-fish. All work on the same principle: a digit confined to a rectangular subset of rows and columns allows eliminations across those lines.

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Frequently Asked Questions

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