Sudoku Glossary: 60+ Sudoku Terms Explained

Sudoku guides, forums and hint messages are full of jargon: peers, nonets, pointing pairs, bilocation, pincers, fish. Each word is simple once someone explains it, but nobody ever does all at once. This glossary collects more than 60 Sudoku terms from A to Z, each with a short, plain definition and a link to a full guide when one exists.

If you are brand new, start with the rules in How to Play Sudoku, which has a seven-word starter glossary. Come back here whenever a guide, a forum post or a hint uses a word you don’t know.

A–B

Band. Three boxes side by side in a horizontal strip: rows 1–3, 4–6 or 7–9. A vertical strip of three boxes is a stack. Bands and stacks matter because a digit must appear once in each of their three boxes.

Bilocation. A digit that has exactly two possible cells in a house. One of the two must hold it, so if one is ruled out the other is confirmed. Two such cells form a conjugate pair, and they are the building blocks of X-Wings and chains.

Bivalue cell. A cell with exactly two candidates left, such as {4,6}. If one candidate is wrong, the other is right. Naked pairs, XY-Wings and XY-Chains are all made of bivalue cells. Don’t mix it up with bilocation: bivalue is two digits in one cell, bilocation is one digit in two cells.

Block. Another word for a box.

Box. One of the nine 3×3 squares outlined in bold. Each box must contain 1 to 9 exactly once. Also called a block, a region or a nonet.

Box-line reduction. Another name for claiming (see C), one of the two kinds of locked candidates.

C

Candidate. A digit that could still go in an empty cell, because no peer already contains it. When people ask “what is a candidate in Sudoku?”, this is it: a possibility, not an answer. Candidates are usually written small, as pencil marks, and solving is mostly the art of crossing them out until one is left.

Cell. One of the 81 small squares in the grid. Each cell belongs to exactly one row, one column and one box.

Centre marks. Pencil marks written in the middle of a cell that list every digit the cell could still be. Compare corner marks. Our guide to Sudoku pencil marks explains both styles.

Chain. A sequence of linked candidates where each link says “if this one is false, that one is true” (or the reverse). Following a chain lets you prove that a candidate far away is impossible. X-Chains use a single digit; XY-Chains hop through bivalue cells. Chains are the main tool for the very hardest puzzles.

Chute. A collective name for a band or a stack: any line of three boxes.

Claiming. If, inside a row or column, a digit can only go in cells that are all in the same box, that line “claims” the digit for that box. You can remove the digit from the other cells of the box. Also called box-line reduction. It is the mirror image of pointing.

Clue. A number printed in the grid at the start. Also called a given. Clues can’t be changed, and the whole solution follows from them.

Column. A vertical line of nine cells. Each column must contain 1 to 9 exactly once.

Conjugate pair. The two cells of a bilocation: the only two places for a digit in a house. Exactly one of them holds the digit.

Corner marks. Small digits written in the corners of a cell, often used only when a digit can go in two or three cells of a box. See Snyder notation.

Cross-hatching. Drawing imaginary lines from every copy of a digit across its row and column to see which cells in a box are left for it. It is the classic way to find a hidden single, and a form of scanning.

D–G

Diagonal Sudoku. A variant where the two long diagonals must also contain 1 to 9 once each. Also called Sudoku X. Read our guide to diagonal Sudoku.

Digit. One of the symbols 1 to 9. Sudoku uses digits only as labels: you never add them up (except in Killer Sudoku), and the puzzle would work exactly the same with nine letters or colours.

Elimination. Removing a candidate from a cell because a rule or a technique proves it can’t go there. Most advanced techniques produce eliminations rather than placements.

Fish. The family of single-digit patterns that includes the X-Wing (2 lines), Swordfish (3 lines) and Jellyfish (4 lines). In each, a digit is confined to the same N crossing lines within N parallel lines, so it can be removed from the rest of those crossing lines.

Given. Another word for a clue: a number filled in by the puzzle setter.

Grid. The whole 9×9 board. A solution grid (or completed grid) is one with every cell filled correctly.

Guessing. Placing a digit without proof and seeing what happens. A well-made puzzle never needs it, and every puzzle on Sudoku.my can be solved by logic alone. See also trial and error.

H–K

Hidden single. A digit that has only one possible cell in a house, even though that cell may have other candidates. It is the first technique most people learn, and the most common move in easy puzzles.

Hidden pair, triple, quad. Two (or three, or four) digits that fit in only the same two (or three, or four) cells of a house. Those cells must hold those digits, so every other candidate in them can be removed. See naked and hidden pairs.

House. Any row, column or box: a group of nine cells that must contain 1 to 9 once each. A standard grid has 27 houses (9 of each). Also called a unit or a group.

Intersection. The three cells where a box overlaps a row or a column. Techniques that use them, pointing and claiming, are known as intersection removal or locked candidates.

Jellyfish. The four-line member of the fish family: a digit confined to the same four columns within four rows (or the reverse). It is rare, and you can solve almost every puzzle without it.

Jigsaw Sudoku. A variant where the nine boxes are replaced by irregular shapes of nine cells. The rules are otherwise the same. It appears in our overview of Sudoku variants.

Killer Sudoku. A variant with few or no givens. Instead, dotted cages show a sum, and the digits in each cage must add up to it without repeating. A key idea is that every house sums to 45. Full guide: Killer Sudoku.

L–M

Line. A row or a column (but not a box). Many techniques are stated in terms of lines, such as “two lines” in an X-Wing.

Locked candidates. The name for both pointing and claiming. A digit is “locked” into the cells where a box meets a line, so it can be removed elsewhere in the box or the line. On Sudoku.my this is the step that makes a puzzle Medium rather than Easy.

Minimal puzzle. A puzzle where removing any one clue would give it more than one solution. Minimal doesn’t mean “fewest possible clues”: different minimal puzzles have different clue counts.

Minimum clues. The smallest number of givens a standard Sudoku can have and still have one solution. It is 17: in 2012, mathematicians Gary McGuire, Bastian Tugemann and Gilles Civario showed by an exhaustive computer search that no 16-clue puzzle exists.

Mini Sudoku. A smaller grid, usually 4×4 (digits 1–4, 2×2 boxes) or 6×6 (digits 1–6, 2×3 boxes). Great for children and for learning the rules.

N–O

Naked single. A cell with only one candidate left, because its peers already contain the other eight digits. Sudoku.my hints call it the “last possible number”.

Naked pair, triple, quad. Two cells in a house that share the same two candidates and nothing else (or three cells holding only three digits between them, and so on). Those digits must go in those cells, so they can be removed from every other cell in the house.

Nishio. A “what if?” test on one digit: assume a candidate is true, follow the consequences for that digit only, and if they lead to a contradiction, remove the candidate. Many solvers see it as a polite form of trial and error.

Nonet. A group of nine cells, most often a 3×3 box. You will also see it used for the irregular regions of Jigsaw Sudoku.

Notes. Another word for pencil marks. In apps, a notes mode lets you tap small candidates into a cell instead of a final answer.

Number Place. The name the modern puzzle was first published under, by Dell Magazines in the United States in 1979. The story is in our history of Sudoku.

P–R

Peer. Any cell that shares a house with a given cell. Every cell has 20 peers: 8 in its row, 8 in its column and 4 more in its box. Two peers “see” each other, which is why they can never hold the same digit.

Pencil marks. Small candidate digits written in a cell while you work. They turn Sudoku from a memory test into a logic puzzle, and every technique from locked candidates onwards needs them.

Pincers. The two outer cells of an XY-Wing (or Y-Wing). Each sees the pivot and both share one digit, which can be removed from any cell that sees both pincers. Also called wings.

Pivot. The middle cell of an XY-Wing: a bivalue cell whose two candidates link to the two pincers. Also called the hinge.

Pointing. If, inside a box, a digit can only go in cells that lie on one row or one column, those cells “point” along that line. You can remove the digit from the rest of the line outside the box. With two cells it is often called a pointing pair.

Proper puzzle. A puzzle with exactly one solution. Puzzles are assumed to be proper unless the setter says otherwise.

r3c5 notation. A short way of naming a cell: r3c5 means row 3, column 5. Forums and solving software use it everywhere. You may also see r3c5<>7 (“r3c5 is not 7”) or r3c5=7 (“r3c5 is 7”), and b5 for box 5.

Region. A box, especially in variants where the boxes aren’t 3×3 squares.

Row. A horizontal line of nine cells. Each row must contain 1 to 9 exactly once.

S

Samurai Sudoku. Five 9×9 grids overlapping in an X shape, where the centre grid shares a corner box with each of the other four. See our Samurai Sudoku guide.

Scanning. Looking along rows, columns and boxes for where a digit can or can’t go. It is how you find singles without writing any notes.

Single. A move that places a digit: either a naked single (one candidate in a cell) or a hidden single (one place in a house). Easy puzzles on Sudoku.my need nothing else.

Snyder notation. A note-taking method associated with Thomas Snyder, three-time World Sudoku Champion: write a digit in a box only when it can go in exactly two cells of that box. It keeps the grid clean and shows conjugate pairs at a glance.

Solution. The completed grid. A proper puzzle has exactly one.

Stack. Three boxes on top of each other in a vertical strip: columns 1–3, 4–6 or 7–9.

Strong link. Two candidates that can’t both be false, so if one is false the other is true. A conjugate pair is a strong link, and so are the two candidates of a bivalue cell.

Swordfish. The three-line fish. A digit sits in two or three cells of each of three rows, and all those cells fall in the same three columns (or the reverse). The digit can be removed from the rest of those three columns. It is covered with its smaller sibling in our X-Wing guide.

Symmetry. Many printed puzzles place their clues in a symmetric pattern, most often so the grid looks the same when rotated 180 degrees. It is a matter of style and has no effect on the logic. Our generator removes clues in mirrored pairs first, so most Sudoku.my puzzles look close to symmetric, but solvability by logic always comes first.

T–Z

Technique. A named, repeatable logical step, like a hidden single or an X-Wing. The hardest technique a puzzle needs is what makes it Easy, Medium, Hard or Expert, more than the number of clues. See Sudoku difficulty levels.

Trial and error. Picking a candidate, filling in what follows and backing up if you hit a contradiction. Also called guessing or bifurcation. It works, but it isn’t logic, and it is never needed on a well-made puzzle.

Uniqueness. The property of having exactly one solution. Some advanced techniques, such as the unique rectangle, assume uniqueness to make eliminations, so they are only safe on puzzles you know are proper.

Unit. Another word for a house: a row, a column or a box.

Weak link. Two candidates that can’t both be true, so if one is true the other is false (but both could be false). Any two candidates of the same digit that see each other form a weak link.

X-Wing. A digit that fits in exactly two cells in each of two rows, with those cells in the same two columns (or the reverse). The digit takes two opposite corners of the rectangle, so it can be removed from the rest of those columns. Full guide: X-Wing Sudoku.

XY-Chain. A chain of bivalue cells, each seeing the next, that starts and ends on the same digit. Any cell that sees both ends can’t hold that digit. An XY-Wing is the shortest XY-Chain.

XY-Wing (Y-Wing). Three bivalue cells: a pivot with candidates XY, one pincer with XZ and another with YZ. Whichever value the pivot takes, one pincer must be Z, so Z can be removed from every cell that sees both pincers. Learn it step by step in our Y-Wing guide.

Which terms do you need first?

You don’t need to learn the whole list before you play. The vocabulary grows with the puzzles, and each difficulty level on Sudoku.my adds a few words:

For the techniques in the order you’ll need them, with a real diagram for each, read 10 Sudoku strategies. And if you just want a puzzle, there’s a new one every day in the daily challenge.

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