The Sudoku Glossary: Every Term Explained

The smallest unit of the game. A single square that holds exactly one digit. There are 81 cells in a standard 9x9 grid. Beginner

A horizontal line of 9 cells spanning the full width of the grid. Each row must contain the digits 1–9 exactly once. Beginner

A vertical line of 9 cells spanning the full height of the grid. Each column must contain the digits 1–9 exactly once. Beginner

One of the nine 3x3 squares outlined by thicker borders. Each block must contain the digits 1–9 exactly once. Beginner

A collective term for any Row, Column, or Block. All three are "houses" — and each must contain 1–9. This term is used heavily in advanced technique descriptions. Beginner

The numbers pre-filled at the start of the puzzle. They are fixed, cannot be changed, and form the foundation of all logical deductions. Beginner

A set of three aligned blocks — either three horizontal blocks (a row-chute) or three vertical blocks (a column-chute). Useful for scanning techniques. Intermediate

The set of possible numbers that could legally fit into a specific cell, based on what is already present in its row, column, and block. Usually recorded as Pencil Marks. Beginner

Small candidate digits written inside a cell to track possibilities. Typically arranged 1–3 on the top, 4–6 in the middle, and 7–9 on the bottom. The essential tool for Medium puzzles and above. Intermediate

A scanning technique where you "project" the rows and columns that already contain a specific number to find the only valid cell in a 3x3 block where that number can go. The most fundamental solving strategy. Beginner

When 8 out of 9 cells in a house are filled, leaving only one possible choice for the last cell. The easiest move in the game — always scan for Full Houses before anything else. Beginner

A cell where only one specific digit can possibly fit because all other eight digits are visible in its row, column, or block. The second most fundamental move after the Full House. Beginner

When a digit appears as a candidate in only one cell within a house, even if that cell has other candidates listed. It must go in that cell because it has nowhere else to go in that house. Beginner

Two cells in the same house that contain the same two candidates — and only those two. This "locks" those numbers, allowing you to safely remove them from the pencil marks of every other cell in that house. Intermediate

Three cells in the same house that collectively contain exactly three candidates between them (e.g., cells with [1,5], [5,9], and [1,9]). Once identified, those three digits can be removed from the rest of the house. Intermediate

Two digits that only appear as candidates in the same two cells within a house, even though those cells have other candidates listed. Those two cells must contain those two digits — all other candidates can be removed from those cells. Intermediate

When all possible locations for a candidate within a 3x3 block fall on the same row or column. That candidate cannot then appear elsewhere in that row or column outside the block. A key bridge from beginner to intermediate solving. Intermediate

The reverse of a Pointing Pair. When all possible spots for a candidate in a row or column fall within a single 3x3 block, that candidate can be eliminated from the rest of that block. Intermediate

A "fish" pattern involving two rows and two columns. A candidate forms a perfect rectangle at four corner cells. Since the candidate must be in one of two diagonal pairs, it can be eliminated from all other cells in those two columns (or rows). Advanced

An X-Wing extended to three rows and three columns. If a candidate appears in only two or three cells across three rows, and all those cells fall within the same three columns, the candidate can be eliminated from those three columns everywhere else. Advanced

An L-shaped pattern involving a "pivot" cell with two candidates [A, B] and two "pincer" cells with [A, C] and [B, C] respectively. Any cell that can "see" both pincers cannot contain digit C. Advanced

Four digits that only appear as candidates in the same four cells within a house. Notoriously hard to spot due to the other pencil marks obscuring the pattern. Once found, all other candidates can be deleted from those four cells. Advanced

When the same number is placed (or appears as a candidate) twice in the same row, column, or block. A conflict always means a mistake has been made somewhere earlier in the solve. Beginner

A board state where a pattern of two digits in four cells would allow for two valid solutions — which violates the fundamental rule that every legitimate Sudoku has exactly one solution. This "uniqueness" principle can actually be used as a valid logical deduction to eliminate candidates. Advanced

The technical term for guessing — picking a candidate and following the logic chain to test it. Also called "trial and error." Most serious solvers avoid this entirely, as every valid Sudoku can be solved without it. Advanced

A restricted pencil-marking method where you only write marks when a candidate can fit in exactly two cells within a block. Keeps grids cleaner than full candidate notation and is ideal for beginners learning to use pencil marks. Intermediate