Question 1

Candidate

Accepted Answer

A digit that could legally go in a cell given the current row, column and box constraints. Also called a pencil mark.

Question 2

Given (Clue)

Accepted Answer

A digit printed in the puzzle at the start. Givens cannot be changed and define the unique solution.

Question 3

Unit

Accepted Answer

Any group of cells that must contain each digit exactly once — a row, a column, or a box (and the diagonals in Sudoku X).

Question 4

Box

Accepted Answer

A 3×3 region of a classic grid (2×3 in 6×6, 2×2 in 4×4) that must contain each digit once.

Question 5

Naked Single

Accepted Answer

A cell with only one remaining candidate, which must therefore be its value.

Question 6

Hidden Single

Accepted Answer

A digit that can be placed in only one cell of a unit, even if that cell shows other candidates.

Question 7

Bi-value Cell

Accepted Answer

A cell with exactly two candidates. Bi-value cells are the building blocks of wings and chains.

Question 8

Strong Link

Accepted Answer

A unit in which a digit has exactly two candidate cells: if one is false the other must be true. The basis of coloring and chains.

Question 9

Locked Candidate

Accepted Answer

A digit confined to the intersection of a box and a line, enabling pointing-pair or box/line-reduction eliminations.

Question 10

Fish

Accepted Answer

A family of single-digit patterns (X-Wing, Swordfish, Jellyfish) defined over N rows and N columns.

Question 11

Pencil Marks

Accepted Answer

Small candidate notes written into a cell to track which digits remain possible. See Snyder notation for an efficient scheme.

Question 12

Uniqueness

Accepted Answer

The property that a valid puzzle has exactly one solution. Unique-rectangle and BUG techniques exploit it.

Question 13

Conjugate Pair

Accepted Answer

A unit in which a digit has exactly two candidate cells, so one of them must be that digit. Conjugate pairs are the strong links that colouring, skyscrapers and W-Wings are built from.

Question 14

Pivot

Accepted Answer

The hinge cell of a wing: in an XY-Wing it is the bi-value cell that sees both pincers; in an XYZ-Wing it carries three candidates and is also part of the elimination.

Question 15

Pincer

Accepted Answer

One of the two outer cells of a wing. Whichever value the pivot takes, one pincer is forced to a shared digit, eliminating it from cells the pincers both see.

Question 16

Chain

Accepted Answer

A sequence of cells linked by alternating strong and weak inferences. Following the chain forces a digit on or off at its ends, the basis of forcing chains and remote pairs.

Question 17

Cross-hatching

Accepted Answer

Scanning the rows and columns that already contain a digit to see which cell of a box is left for it — the fastest way to find hidden singles.

Question 18

Naked Subset

Accepted Answer

A group of N cells in a unit whose combined candidates total exactly N digits (pair, triple or quad), locking those digits out of the rest of the unit.

Question 19

Hidden Subset

Accepted Answer

N digits restricted to the same N cells of a unit (pair, triple or quad), so every other candidate can be erased from those cells.

Question 20

Deadly Pattern

Accepted Answer

A candidate arrangement that would allow two different solutions. Because a real puzzle is unique, such a pattern can never complete — uniqueness techniques eliminate the candidate that would create it.

Question 21

Cage

Accepted Answer

In Killer Sudoku, a dotted group of cells with a target sum. Its digits must add to the total and never repeat — the source of all killer-specific deductions.

Question 22

45 Rule

Accepted Answer

A Killer Sudoku tactic: since every row, column and box sums to 45, adding the cages in a region reveals a single leftover cell (an innie) or protruding cell (an outie).

Question 23

Region

Accepted Answer

The nine-cell group that replaces the 3×3 box in Jigsaw Sudoku. It can be any connected shape, but like a box it must still contain 1–9 once.

Question 24

Diagonal

Accepted Answer

In Sudoku X, each of the two main diagonals is an extra unit that must contain 1–9 once. The centre cell lies on both.

Question 25

Scanning

Accepted Answer

Searching a unit or the whole grid for forced placements — usually for hidden singles by digit — before resorting to pencil marks or advanced techniques.

Question 26

Elimination

Accepted Answer

Removing a candidate from a cell because the logic rules it out. Most techniques beyond singles work by elimination, narrowing cells until a single remains.

Sudoku Glossary