Every term you’ll encounter in a sudoku discussion, defined in plain language. Tagged by where they come up (Basics / Intermediate / Advanced / Notation), linked to the relevant deeper guide where one exists.
Bivalue cell
Intermediate
A cell with exactly two possible candidates. Bivalue cells are the raw material for Y-wings and other chain patterns — knowing both candidates collapses to one as soon as you can eliminate either.
One of the nine 3×3 regions bordered by thicker lines. Each box must contain digits 1-9 exactly once — that's the third sudoku rule (alongside row and column).
Box-line reduction
Intermediate
The cousin of pointing pairs, going the other direction. If a digit's candidates in a row (or column) are all confined to a single 3×3 box, that digit must be in those cells — so it can't appear anywhere else in the box. Eliminate accordingly.
A digit that could legally go in a specific empty cell given the current state of the board. Players often write candidates in cells as pencil marks. A naked single is a cell with exactly one candidate.
Chain
Advanced
A connected sequence of cells linked by candidate logic. XY-chains, AIC (alternating inference chains), and forcing chains are all advanced techniques used on Extreme puzzles — they let you trace 'if this is X then that is Y then …' to derive eliminations.
Cross-hatching
Basics
The fundamental scanning technique. For a given digit, look at rows and columns containing it to figure out which cell in a target box must hold the digit. Most easy puzzles fall entirely to cross-hatching plus naked singles.
The hardest difficulty in Melio Sudoku. 17 starting clues (the proven minimum for a uniquely solvable sudoku). Requires advanced techniques like swordfish, jellyfish, and forcing chains.
Family of patterns where a digit is restricted to N cells across N rows (or columns) that share N columns (or rows). X-wing is N=2, swordfish is N=3, jellyfish is N=4. Each lets you eliminate the digit from outside the pattern's column or row set.
A digit pre-filled into the puzzle by the constructor. Givens can't be changed. Easy puzzles have 30-35 givens; Extreme has 17.
Hidden pair
Intermediate
Two digits that can only go in two specific cells within a single row, column, or box — even though those cells have other candidates as well. Those two cells must hold the pair, so all OTHER candidates can be erased from them.
A digit that has only one legal cell within a row, column, or box — even if that cell has other candidates. Place the digit; clear the other candidates from that cell.
Naked pair
Intermediate
Two cells in the same row, column, or box whose candidate sets are exactly the same two digits. Those two cells must hold those two digits between them, so the pair can be eliminated as candidates from every OTHER cell in the unit.
A cell with exactly one legal candidate. The simplest and most common 'next move' on easy and medium puzzles. /sudoku/practice drills these in burst form.
Most sudoku discussions reference cells as 'r3c5' (row 3, column 5). Rows are numbered 1-9 top-to-bottom; columns 1-9 left-to-right. A range like 'r3c5-7' means r3c5, r3c6, r3c7.
Peer
Basics
Any cell that shares a row, column, or 3×3 box with a given cell. A cell has 20 peers (8 row + 8 column + 8 box, minus overlaps). Placements affect candidates in all peer cells.
Pencil marks
Basics
Small candidate digits written into empty cells to track which placements are still possible. Essential for hard+ puzzles. On Melio, the in-game pencil-mark toggle (key 'N' or the Notes button) flips into note-entry mode.
Pointing pair / triple
Intermediate
When a digit's candidates inside a 3×3 box are all in the same row (or column), the digit can't appear anywhere else in that row (or column) outside the box. The easiest intermediate technique.
One of the nine horizontal lines on the board. Each row must contain digits 1-9 exactly once.
Scanning
Basics
The act of visually sweeping rows, columns, and boxes to find forced placements. Strong scanners can solve Easy puzzles in under 3 minutes without ever writing pencil marks.
Skyscraper
Advanced
Two strong links on the same digit, sharing a single column (or row). The 'tops' of the skyscraper — the non-shared ends — eliminate the digit from any cell that sees both. Easier to spot than X-wing in many cases.
A digit that has exactly two candidate positions in a row, column, or box. The digit MUST be in one of those two cells. Strong links are the building blocks of every fish pattern and many chain techniques.
Unique rectangle
Advanced
A pattern of four cells forming a rectangle across two rows + two columns + two boxes, all sharing the same two candidates. If left unconstrained, the puzzle would have two valid solutions — since sudoku puzzles must be unique, at least one of those four cells has to have a different value, which forces eliminations.
Generic name for a row, column, or 3×3 box — the three groupings that each must contain digits 1-9. Most sudoku rules and techniques are stated in terms of units rather than enumerating row/col/box separately.
X-wing
Advanced
A digit restricted to exactly two cells in each of two rows, where those cells share the same pair of columns (forming a rectangle). Eliminates the digit from the rest of those two columns. The gateway to advanced solving.
Three bivalue cells in a chain: pivot {A,B} + wing {A,C} + wing {B,C}, where each wing sees the pivot. The shared candidate C can be eliminated from any cell that sees both wings. Different from X-wing despite the similar name.