Sudoku glossary

This glossary defines the words used across Sudoku Online, from basic terms such as cell, house and candidate to technique names such as X-Wing and unique rectangle. Each term is defined once, in plain language, and linked to the page that uses it most.

Cells are written r3c7, meaning row 3, column 7, counted from the top and from the left.

Board and rules

Cell: One square of the grid. A classic board has 81. A cell holds a value or pencil marks. See the rules.
Given: A digit that is filled in when the puzzle starts. Givens are locked and cannot be changed. See the rules.
Value: The digit you enter in a cell as your answer, as opposed to a pencil mark. See the controls.
Row and column: A horizontal line of cells is a row, and a vertical line is a column. Rows are counted from the top and columns from the left. See the rules.
Box: One of the nine 3×3 blocks of a classic board. On smaller and larger boards the boxes have other shapes, such as 2×2, 2×3 or 4×4. See the rules.
Region: A group of cells that must contain each digit once, used as a general word for boxes and for the jigsaw shapes of the irregular board. See irregular Sudoku.
House: Any group of cells that must contain each digit exactly once: a row, a column, a box or region, or in a variant an extra group such as a diagonal. This site uses “house” in technique lessons. See how to solve Sudoku.
Peer: A cell that shares a house with another cell. A digit placed in a cell cannot appear in any of its peers. A classic cell has 20 peers. See the TypeScript solver tutorial.
Unique solution: The property of having exactly one solution. Every puzzle here has it, and a check repeated before each release confirms it. See how puzzles are generated and verified unique.
Conflict: A digit that repeats within a house or breaks a variant’s extra rule. See common mistakes.
Wrong digit: A value that differs from the puzzle’s one solution. It may not conflict with anything yet. See common mistakes.

Candidates and notes

Candidate: A digit that could still go in an empty cell, because it does not appear in any of the cell’s peers. See the pencil marks guide.
Pencil mark or note: A small digit written in an empty cell to record a candidate. A cell holds either a value or notes, never both. See the pencil marks guide.
Auto-notes: A button that fills every empty cell’s notes from the digits already placed. It does not remember eliminations made by reasoning. See controls.
Digit-first mode: An entry mode in which you arm a digit on the pad and then tap cells to place it. See controls.
Elimination: Crossing out a candidate that cannot be right. Most techniques above singles produce eliminations and not placements. See how to solve Sudoku.
Placement: Writing a value into a cell after a deduction has settled it. See how to play Sudoku.

Naked and hidden sets

Naked: A pattern that is visible on the cells themselves, such as a cell with one candidate or two cells with the same two candidates. See naked singles.
Hidden: A pattern that is visible only when you look at where a digit can go in a house, so the cells may carry other candidates that disguise it. See hidden singles.
Naked single: A cell with exactly one candidate left. See naked singles.
Hidden single: A digit that has only one possible cell left in a row, column or box. See hidden singles.
Naked pair: Two cells in one house that share exactly the same two candidates. See naked pairs.
Hidden pair: Two digits that fit in only the same two cells of a house. See hidden pairs.
Naked triple: Three cells in one house whose candidates, taken together, are only three digits. See naked triples.

Lines, boxes and fish

Pointing pair: A digit that, inside one box, can go only in two or three cells on one row or column, which lets you remove it from the rest of that line. See pointing pairs.
Box/line reduction: The reverse of a pointing pair: a digit confined, within a row or column, to cells inside one box, which lets you remove it from the rest of that box. See box/line reduction.
Fish: A family of patterns that follow one digit across several rows and the same number of columns. X-Wing and Swordfish are fish. See X-Wing.
X-Wing: A digit with exactly two possible cells in each of two rows, lined up in the same two columns. See X-Wing.
Swordfish: The X-Wing idea extended to three rows and three columns. See Swordfish.

Wings, chains and uniqueness

Bivalue cell: An empty cell with exactly two candidates. XY-Wings are built from them. See XY-Wing.
XY-Wing: Three bivalue cells, a pivot and two wings, which together force a shared digit into one of the wings. See XY-Wing.
Pivot and wing: In an XY-Wing, the pivot is the cell that sees both of the other two cells, which are the wings. See XY-Wing.
Strong link: A house in which a digit has exactly two possible cells, so one of them must be true. See simple coloring.
Simple coloring: Coloring the cells of a chain of strong links alternately for one digit, so that exactly one color is true. See simple coloring.
Unique rectangle: A deduction that relies on the puzzle having one solution, used when three corners of a rectangle share the same two candidates. See unique rectangles.

Variant terms

Cage: In killer Sudoku, a group of adjacent cells whose digits must add up to a target sum and may not repeat. See killer Sudoku.
Diagonal: In diagonal Sudoku, either of the two long lines of the board that must also contain each digit once. See diagonal Sudoku.
Window: In hyper Sudoku, one of four extra 3×3 areas that must contain each digit once. See hyper Sudoku.
Jigsaw region: An irregular, connected group of nine cells that replaces a box on the irregular board. See irregular Sudoku.
Knight’s move: Two cells in one direction and one at a right angle, as a chess knight moves. In anti-knight Sudoku, cells a knight’s move apart cannot share a digit. See anti-knight Sudoku.
Daily puzzle: The single puzzle shared by all players for each UTC day. See daily Sudoku.

Site terms

Technique: A named pattern that lets you place a digit or remove a candidate with certainty. The site’s solver knows twelve for classic play. See the techniques hub.
Level or tier: One of Easy, Medium, Hard, Expert and Evil, set by the hardest technique a puzzle needs. This is the site’s own grading. See how difficulty is graded.
Hint levels: The three stages of a hint: where to look, which technique, and the move. See controls.
Solution counter: A search that counts a puzzle’s solutions and stops at two, which is enough to tell whether it is unique. See how puzzles are generated and verified unique.
Backtracking: A search that tries a digit, continues, and backs out when it reaches a dead end. See the TypeScript solver tutorial.
Minimum remaining values: The search rule of always branching on the empty cell with the fewest candidates. See the TypeScript solver tutorial.