Sudoku Glossary

Every term you'll meet on Sudoku.by - from basic grid vocabulary to technique names and variant clues - defined in plain language. Where a term has a full guide, the definition links to it. Looking for strategy instead? See How to Solve Sudoku.

0–9

45 Rule

Because every row, column, and box must contain the digits 1–9 exactly once, each one sums to 45. The 45 rule is the arithmetic backbone of Killer Sudoku, where comparing cage totals against 45 reveals the value of innies and outies.

A

Anti-Knight

A constraint borrowed from chess: no two cells a knight's move apart may hold the same digit. Try it in our Anti-Knight Sudoku puzzles.

Arrow Sudoku

A variant where the digits along an arrow's shaft must add up to the digit in the circle at its base. Play it on the Arrow Sudoku hub.

B

Bi-Value Cell

A cell with exactly two candidates remaining. Bi-value cells are the building blocks of advanced moves like the Y-Wing and most chain techniques.

Box

One of the nine 3×3 regions of the grid, each of which must contain 1–9 exactly once. Also called a block, nonet, or region.

C

Cage

A dotted-outline group of cells in Killer Sudoku whose digits must add up to the small printed total, with no digit repeating inside the cage.

Candidate

Any digit that could still legally go in a given empty cell. Solving is largely the art of eliminating candidates until one remains.

Chute

A band of three boxes in a row (a horizontal chute) or a stack of three boxes in a column (a vertical chute). The grid has three of each.

Conjugate Pair

The only two cells in a unit that can hold a particular digit. Exactly one of them must contain it - a strong link exploited by Skyscrapers, X-Wings, and colouring techniques.

Crosshatching

Eliminating digits from a cell by checking its row, column, and box - the manual scan behind the Last Possible Number technique.

F

Fish

The family of patterns where one digit's candidates are confined to a matching set of rows and columns. The two-line fish is the X-Wing, the three-line fish is the Swordfish, and the four-line fish is the Jellyfish.

G

German Whisper

A line variant where every two adjacent digits on the line must differ by at least 5 - so a 5 can never appear on the line at all. Play it on the German Whispers hub.

Given

A digit printed in the grid before you start, also called a clue. A classic 9×9 Sudoku needs at least 17 givens to have a unique solution.

H

Hidden Pair

Two digits that appear as candidates in only the same two cells of a unit, hidden among other candidates. All non-pair candidates can be erased from those cells - see the full Hidden Pairs guide.

Hidden Single

A digit that has only one legal cell in a unit, even though that cell still shows several candidates. Spotting Hidden Singles is the key skill for Medium puzzles.

House

A synonym for unit: any row, column, or box that must contain the digits 1–9 exactly once.

I

Innie & Outie

In Killer Sudoku, an innie is a cell poking into a region whose other cage sums are known, and an outie pokes out of it. Comparing cage totals against multiples of 45 gives their exact values - explained in the Killer 45 rule guide.

J

Jellyfish

The four-row, four-column member of the fish family. Extremely rare in practice - almost everything a Jellyfish solves can be solved by smaller fish first.

K

Killer Sudoku

A popular variant where cages with sum clues replace most or all givens, blending classic logic with arithmetic. Start at the Killer Sudoku hub.

Kropki Dot

A dot between two cells in Kropki Sudoku: a white dot means the digits are consecutive, a black dot means one is exactly double the other.

L

Last Free Cell

The simplest move in Sudoku: when a unit has just one empty cell, fill it with the missing digit. See the Last Free Cell guide.

Locked Candidates

When all of a digit's candidates in a box fall on a single row or column (or vice versa), the digit is "locked" there and can be eliminated from the rest of that line or box. Pointing pairs are the most common form.

N

Naked Pair

Two cells in a unit that each contain only the same two candidates, locking those digits in place and removing them from the unit's other cells. Covered in full as Obvious Pairs.

Naked Single

A cell whose candidate list has shrunk to exactly one digit, which can be placed immediately. Covered in full as Obvious Singles.

Naked Triple

Three cells in a unit that collectively contain only three candidates between them. Those three digits can be eliminated from every other cell in the unit.

P

Pencil Mark

A small digit written in a cell to record a remaining candidate - called notes in digital play. Our guide to Notes in Sudoku covers when and how to use them.

Pointing Pair

When a digit's only candidates in a box sit in one row or column, they "point" along that line: the digit can be removed from the rest of the line outside the box.

R

Renban Line

A line that must contain a set of consecutive digits - such as 4-5-6-7 - in any order. Play this variant on the Renban Sudoku hub.

S

Sandwich Clue

An outside clue giving the sum of the digits "sandwiched" between the 1 and the 9 in that row or column. The defining clue of Sandwich Sudoku.

Scanning

Systematically sweeping rows, columns, and boxes - usually digit by digit - to find forced placements. The opening move of every solve; see How to Solve Sudoku.

Skyscraper

As a variant, Skyscraper Sudoku treats digits as building heights, with outside clues counting how many buildings are visible. The same word also names a chain technique built from two conjugate pairs.

Snyder Notation

A minimalist pencil-marking style, popularised by champion Thomas Snyder, where you note a candidate only when it is restricted to exactly two cells within a box. Faster than full notation for competitive solving.

Swordfish

A fish pattern in which one digit is confined to the same three columns across three rows (or the reverse), enabling eliminations in the covering lines. Full walkthrough in the Swordfish guide.

T

Thermometer

A grey line in Thermo Sudoku along which digits must strictly increase, starting from the bulb end.

U

Unique Rectangle

A technique that exploits the fact that a valid puzzle has exactly one solution: any candidate arrangement that would create two interchangeable solutions (four corners sharing the same two candidates across two boxes) must be avoided, forcing an elimination.

Unit

Any group of nine cells that must contain 1–9 exactly once: a row, a column, or a box. Variants add extra units such as diagonals or cages.

W

Whisper Line

Any line constraint requiring adjacent digits to differ by at least a fixed amount. The best-known is the German Whisper with a minimum difference of 5 - try it on the German Whispers hub.

X

X-Wing

The two-row, two-column fish: a digit confined to the same two columns in each of two rows forms a rectangle, eliminating that digit from the rest of those columns. Step-by-step in the X-Wing guide.

XV Sudoku

A variant where an X between two cells means their digits sum to 10 and a V means they sum to 5 - and, in the strict ruleset, the absence of a mark forbids both sums. Play it on the XV Sudoku hub.

Y

Y-Wing

A three-cell pattern of bi-value cells: a pivot cell sees two "pincer" cells, and whichever way the pivot resolves, one pincer must contain the shared digit - so that digit can be eliminated from every cell both pincers see.