Sudoku Terms

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Published April 28, 2026 · Updated October 20, 2026

Sudoku has a small vocabulary, and most guides use it without stopping to explain. This glossary defines each term in a line or two and links the page on this site that goes deeper. Where one of our technique guides measured something that changes the usual definition, the entry here says so. Where a term is a button in our games, it is spelled the way the button is.

What are the parts of a sudoku grid called?

• Square, or cell. One of the 81 spaces. Our guides say square; most solving software says cell.
• Row and column. Nine of each, numbered 1 to 9 from the top and from the left.
• Box, or block. One of the nine 3×3 squares outlined in bold, numbered 1 to 9 in reading order, so box 5 is the centre one. Wikipedia’s glossary notes that boxes are also called blocks or regions (Glossary of Sudoku).
• Region. Any outlined shape that must hold each digit once. In classic sudoku the regions are the boxes; in jigsaw sudoku they are irregular.
• House, or unit. A row, a column or a box: any group of nine that must hold 1 to 9 once. A classic grid has 27.
• Band and stack. A band is three boxes side by side (rows 1–3, 4–6 or 7–9), a stack three boxes one above the other. The SudoCue glossary calls either one a chute (SudoCue).
• Peer. A square that shares a house with another. Every square has 20: the other eight squares of its row and of its column, plus the four squares of its box that lie in neither. Peers are said to see each other, so they never share a digit.
• Given, or clue. A digit printed at the start, which never changes. At the first move our classic levels deal a median of 40 givens on Easy, 32 on Medium, 26 on Hard and 23 on Expert.
• R5C5. Row first, then column: R5C5 is the centre square, R1C9 the top right. Every diagram on this site names squares this way.

What is a candidate in sudoku?

• Candidate. A digit that none of a square’s peers holds yet, so it could still be the answer. Written small in the square, candidates are pencil marks, or notes, the name of the button in our games. Our guide to sudoku pencil marks compares full marks with the lighter corner marks.
• Placement and elimination. A placement writes a digit in for good; an elimination crosses a candidate out. Every technique below does one or the other, and most of them only eliminate.
• Bivalue square. A square down to two candidates, like R5C5 above. Wings, remote pairs and XY-chains are built from them. A square with three is trivalue.
• Conjugate pair, or strong link. A digit with exactly two places left in a house, such as the 5s of column 5 above. If one place is not the digit, the other is. SudoCue calls it a bilocation. Not the same thing as a naked pair, which is two squares sharing two digits.
• Weak link. Two places that cannot both hold the same digit, because they see each other. At most one of them is the digit, and possibly neither. Chains alternate strong and weak links.
• Contradiction. A state the rules forbid: a square with no candidate left, a house with no place left for a digit it needs, or two squares of a house that can each only take the same digit. A contradiction means an earlier digit or elimination was wrong. Our guide to sudoku mistakes measured how long that can take to show.

What do sudoku technique names mean?

Each name below links its guide on this site, and each definition is the one that guide works from. The families run roughly from easiest to hardest.

Singles

• Naked single: a square with one candidate left. Also called a sole candidate; when it is the last empty square of a house, a full house.
• Hidden single: a digit with one place left in a house, as the 8 of row 5 is in R5C5 above, even though that square still shows two candidates. Found by cross-hatching; Sudopedia also calls it a pinned digit.

Locked candidates

• Locked candidates: a digit confined to the three squares where a box crosses a row or column. Two kinds, by where you start:
• Pointing pair or triple (type 1): a box’s places for a digit all lie on one line, so the digit leaves the rest of that line.
• Box/line reduction, or claiming (type 2): when one box holds every spot a row or column has left for a digit, the box loses that digit everywhere off the line.

Subsets

• Naked pair: two squares in a house with the same two candidates and no others. Those digits leave every other square of the house. When the two share a box as well as a line, some call it a locked pair.
• Naked triple: three squares of a house whose candidates come to three digits in all. No square needs all three; {1,2}, {2,3} and {1,3} is a triple. Four squares and four digits make a quad.
• Hidden pair and hidden triple: two (or three) digits whose only places in a house are the same two (or three) squares. Those squares lose every other candidate; nothing else in the house changes.

Fish

• X-Wing: one digit, two rows with exactly two places each, in the same two columns. The digit leaves the rest of both columns. It works the same with rows and columns swapped.
• Swordfish and Jellyfish: the same with three lines or four. A line need not hold the digit in every one of the columns.
• Finned X-Wing and finned Swordfish: a fish with one or two extra places, the fin, all in one box. The digit then goes only where the cover lines, the columns of a fish on rows, run through the fin’s box.

Single-digit chains

• Skyscraper: two parallel strong links on one digit whose ends line up at one side only. One of the two loose ends, the roofs, holds the digit, so a square that sees both loses it.
• Simple colouring: joining one digit’s strong links into clusters and colouring them alternately. One colour is the digit throughout, and the rules work without knowing which.
• X-chain and X-cycle: a chain of strong and weak links on one digit, open or closed into a loop. A closed one is a nice loop; an open chain on any number of digits is an AIC (alternating inference chain). Turbot fish is a name for some four-square X-chains.

Wings and bivalue chains

• XY-Wing, also Y-Wing: a bivalue pivot holding X and Y, seeing two bivalue pincers holding X and Z and Y and Z. One pincer is Z, so Z leaves any square that sees both.
• XYZ-Wing: the pivot holds all three digits, and Z leaves only squares that see all three cells. Its guide proves that can be at most two squares, and the pincers must not see each other, or the three are a naked triple.
• W-Wing: two bivalue squares with the same two digits that do not see each other, plus a strong link on one digit whose ends each see one of them. The other digit leaves any square that sees both.
• Remote pairs: a chain of squares that all hold the same two digits, each seeing the next. Two chain squares an odd number of steps apart hold both digits between them, so a square that sees both loses both. Squares an even number apart prove nothing.
• XY-chain: a chain of bivalue squares, each sharing a digit with the next, that starts and ends on the same digit. A square that sees both ends loses that digit.

Sets, colouring and uniqueness

• Almost locked set (ALS): squares of one house holding one candidate more than there are squares; a single bivalue square is the smallest. ALS-XZ joins two of them through a restricted common digit, X, and removes a second shared digit, Z.
• Sue de Coq: two or three squares where a line crosses a box, holding two digits more than squares, with one partner on the line and one in the box. Each partner’s digits leave its own house: the line partner’s the line, the box partner’s the box.
• 3D Medusa: simple colouring across digits, linking candidates through bivalue squares as well as through houses.
• Unique rectangle: four squares on two rows, two columns and two boxes that would be a deadly pattern, four squares holding the same two digits that could swap. A puzzle with one answer can never reach one, so the pattern is avoided. It only works if the puzzle really has one answer.
• BUG, the bivalue universal grave: every open square bivalue and every remaining digit twice in each house. A one-answer puzzle never ends there, so in BUG+1, where the board is one candidate short of a BUG, that one candidate is the answer.
• Forcing chain: following every option of a square or a digit until all of them agree. A Nishio, or contradiction chain, follows one candidate until the board breaks, which removes it.

What do unique, minimal and 17-clue mean?

• Proper, or well-posed. A puzzle with exactly one solution, which Wikipedia’s glossary calls a proper sudoku. Every board our classic generator deals has one, because each digit it takes away is checked: its own logic must still finish the board. Every line of the Evil and Extreme banks has been counted to exactly one solution.
• Minimal, or irreducible. A proper puzzle that stops being proper if any one given is removed. None of 200 boards from our Easy level is minimal, nor any of 200 from Medium: each could lose a given and keep one answer. On Hard 24 of 200 are, and on Expert 179 of 200.
• 17-clue. The fewest givens a classic sudoku with one solution can have. In 2012 Gary McGuire, Bastian Tugemann and Gilles Civario proved by exhaustive computer search that 16 givens are never enough; our 17-clue sudoku page deals real ones.
• Difficulty level. Sites use the same names for different things. On ours a level is set by what it takes to finish the board, not by how many givens it has; our guide to sudoku difficulty levels compares them.
• Trial and error, guessing, backtracking. Trying a digit to see what happens and taking it back if the board breaks. Computer solvers search this way. A forcing chain is the same idea done carefully by hand; none of our Easy to Expert boards needs either.

What do the buttons in our sudoku games mean?

• Notes switches the number buttons to pencil marks; Start Notes on the classic pages, Auto Notes on the themed games and Fill on Kropki sudoku write every candidate in at once.
• Mistakes counts every wrong digit placed. It never goes down and there is no limit.
• Hint puts one right digit into a square that is empty or wrong (Kropki’s explains a step instead). Solve fills in the whole answer.
• Undo and Erase are on every game, New Game on all but the daily puzzle, and Redo on the classic pages.
• In the Sudoku Helper, Next Hint names the simplest technique that applies, Apply Hint makes the move, and Available Techniques lists everything else that applies at that moment.
• In the Sudoku Trainer, you pick a technique, click the pattern’s squares and press Check; Hint gives part of it away and Find Pattern shows the whole thing.
• In the Sudoku Solver, Solve fills the grid, Step fills in one square at a time, in reading order, and Import and Export read and write an 81-character puzzle string.

What do variant terms like cage and thermometer mean?

• Cage and cage total (killer sudoku): a dashed group of squares whose digits add up to the total in its corner, with no digit twice.
• Thermometer (thermo sudoku): digits rise strictly from the round bulb to the tip.
• Arrow and circle (arrow sudoku): the circled digit is the sum of the digits along its arrow.
• Kropki dots (kropki sudoku): a white dot joins digits one apart, a black dot a digit and its double, and a border with no dot is neither.
• X and V (XV sudoku): neighbours summing to 10 and to 5, and only where a mark is drawn. Bars (consecutive sudoku) do the same for digits one apart.
• Sandwich clue (sandwich sudoku): the total of the digits sandwiched between a row’s or column’s 1 and 9.
• Skyscraper clue (skyscraper sudoku): how many digits, read as building heights, can be seen from the edge.
• Whisper line (German Whispers): neighbours along the line differ by at least 5. Renban group (renban sudoku): a group whose digits form a consecutive run in any order.
• Knight’s move (anti-knight sudoku): two squares a chess knight’s jump apart may not hold the same digit.
• Diagonals and windows: the two long diagonals of X sudoku, and the four extra 3×3 windows of Hyper sudoku, also sold as Windoku and NRC sudoku, must each hold 1 to 9 too.

Our list of sudoku variations covers every variant we host, grouped by the rule each one adds.

What do sudoku abbreviations stand for?

The short forms our guides and diagrams use. Rows count from the top, columns from the left and boxes in reading order.