The basics
Sudoku glossary
Sudoku has its own jargon. Here is what each term means, in one or two sentences.
A glossary of the words you meet in Sudoku guides: cell, unit, box, candidate, given, notes, naked and hidden sets, pointing pairs, fish and chains. Each entry is short, and technique entries link to a full guide.
Grid terms
Cell
One of the 81 squares of the grid. Cells are named by row and column, for example R3C9 for row 3, column 9.
Row, column, box
The three kinds of groups that must each contain 1 to 9 once. A box is a 3 by 3 block, and there are nine of them.
Unit
Any row, column or box. There are 27 units in a grid.
Peers
All cells that share a unit with a given cell: 20 cells in total.
Given (or clue)
A digit already printed in the puzzle at the start. Givens cannot be changed.
Candidate
A digit that could still go in an empty cell. Writing candidates down is called making notes or pencil marks.
Techniques
Naked single
A cell with only one candidate left. Read more.
Hidden single
A digit that has only one possible cell left in a unit. Read more.
Naked pair, triple
Two (or three) cells in a unit that together contain only two (or three) candidates. Those digits can be removed from the other cells of the unit. Pairs, triples.
Hidden pair, triple
Two (or three) digits that fit in only the same two (or three) cells of a unit. Other candidates in those cells can be removed. Pairs, triples.
Pointing pair
A digit in a box that sits in just one row or column of that box. It can then be removed from the rest of that row or column. Read more.
Box-line reduction
The reverse: a digit in a row or column that sits in just one box. Read more.
Fish (X-Wing, Swordfish)
A digit locked into the same two (X-Wing) or three (Swordfish) lines across the grid. X-Wing, Swordfish.
XY-Wing
Three cells with two candidates each, linked in a chain, that remove a candidate from cells seeing two of them. Read more.
Coloring
Following a digit through a chain of strong links, alternating two colors. Read more.
Puzzle terms
Unique solution
A proper Sudoku has exactly one solution. Read more.
Killer Sudoku
A variant with cages that have a target sum. Read more.
Strong link
Two cells that are the only places for a digit in a unit. If one is false, the other must be true.
Frequently asked questions
What is a unit in Sudoku?
A row, a column or a 3 by 3 box. Each grid has 27 units.
What is a candidate?
A digit that could still be placed in an empty cell.
What is a given?
A digit already printed in the puzzle at the start.