Sudoku Glossary

Updated 15 October 2026

Sudoku terms are the grid vocabulary — cell, row, column, box, candidate, given — plus every named solving technique, from full house to forcing chain, that justifies a placement or an elimination. This glossary defines all 31 of those techniques plus the core vocabulary, matching the how to play guide and the technique index.

Grid basics

The parts of the board that every technique talks about.

Cell: One of the 81 positions in the grid, holding either a filled digit or, while blank, a set of candidates.
Row: One of the nine horizontal lines of nine cells. Every row must contain the digits 1 to 9 exactly once.
Column: One of the nine vertical lines of nine cells. Every column must contain the digits 1 to 9 exactly once.
Box (unit): One of the nine 3×3 blocks of cells. Unit is the general term for any row, column or box.
Candidate (pencil mark): A digit a blank cell could still legally hold, based on what its row, column and box already contain.
Given (clue): A digit printed on the board before solving starts, part of what makes one puzzle different from another.

Solving concepts

The ideas that explain why a technique works, rather than what it looks like.

Naked / hidden: Naked patterns are visible in a cell’s own candidates; hidden patterns need checking, unit by unit, for a digit.
Elimination: Removing a candidate from a cell because a technique has proven it cannot be the answer, without placing a digit.
Unique solution: The property every well-formed Sudoku has: exactly one completed grid satisfies all three rules given its clues.
Minimal puzzle (17-clue): A puzzle with the fewest clues that still has a unique solution — 17 for standard Sudoku.

Singles and the last cell

The simplest placements: each one fills a cell outright instead of removing candidates. Techniques are listed in the difficulty order Suuudokuuu applies them, and every one has its own guide with a worked example board built from the real solving engine.

Full House: The last empty cell in a row, column or box. It must take the one digit that unit is still missing.
Naked Single: A cell whose row, column and box between them eliminate every candidate but one.
Hidden Single: A digit that fits in only one cell of a unit, even though that cell still carries other candidates.

Intersections

Box and line overlaps that confine a digit to a single line or box and erase it elsewhere.

Pointing Pair: A digit confined to two cells of a box sharing a row or column, erased from the rest of that line.
Pointing Triple: The same pattern as a pointing pair, with three cells confined to a box-line intersection instead of two.
Box Line Reduction: The mirror of a pointing pair: a digit confined to one box along a line is erased from the rest of that box.

Subsets

Groups of cells or digits that lock each other inside one unit, in naked and hidden forms.

Naked Pair: Two cells sharing the same two candidates and nothing else, so both digits are erased elsewhere in the unit.
Naked Triple: Three cells in a unit whose candidates together use only three digits.
Naked Quad: Four cells in a unit whose candidates together use only four digits.
Hidden Pair: Two digits that fit in only two cells of a unit, which reserves those cells for the pair.
Hidden Triple: Three digits confined between them to the same three cells of a unit.
Hidden Quad: Four digits confined between them to the same four cells of a unit.

Fish

Patterns where one digit is confined to the same few lines across the same number of rows and columns.

X-Wing: A fish pattern: a digit confined to the same two columns across two rows is erased from the rest of those columns.
Swordfish: The three-line version of an X-Wing, using three rows and three columns instead of two.
Jellyfish: The four-line version of the same fish pattern, using four rows and four columns.
Finned X-Wing: An X-Wing with an extra candidate, called a fin, that still supports a smaller set of eliminations near the fin.
Finned Swordfish: A Swordfish pattern with fins, the three-line counterpart to a finned X-Wing.
Sashimi X-Wing: A finned X-Wing where removing the fin cell’s own candidate entirely still leaves a valid, smaller X-Wing behind.
Sashimi Swordfish: The same sashimi pattern applied to a Swordfish instead of an X-Wing.

Wings

Short chains of bivalue cells that force an elimination whichever way the pivot resolves.

XY-Wing: Three bivalue cells where, whichever value the pivot holds, a shared candidate is erased from cells seeing both others.
XYZ-Wing: An XY-Wing where the pivot cell also carries the shared third candidate, which tightens the elimination further.
W-Wing: Two cells sharing a candidate pair, linked by a strong link on one digit, that erases the other from shared peers.

Chains and coloring

Longer links of strong and weak inferences, from a single digit to a mixed alternating chain.

X-Chain: A chain of strong and weak links on one digit connecting two cells, proving an endpoint of the chain must hold it.
XY-Chain: A chain of bivalue cells linked so the two endpoints force an elimination wherever their shared peers overlap.
Simple Coloring: Assigns two alternating colors to a chain of strong links on one digit, then clears candidates seeing both colors.
AIC (Alternating Inference Chain): A chain of alternating strong and weak links across candidates, generalising X-Chains, XY-Chains and coloring.

Uniqueness and forcing chains

Patterns that lean on the single-solution rule, and trial branches that keep only what every branch agrees on.

Unique Rectangle: Three corners of a rectangle in two boxes hold the same pair, so the fourth corner cannot take either digit of it.
Bivalue Universal Grave: BUG+1: every empty cell is bivalue except one, which takes the digit appearing three times in its units.
Nishio Forcing Chain: Assumes one candidate and follows the singles it forces; a contradiction proves the candidate false.
Cell Forcing Chain: Tries every candidate of one cell and keeps any placement or elimination all of the branches agree on.
Region Forcing Chain: Tries every cell where a digit can go in one unit and keeps whatever all of the branches agree on.

Sudoku glossary FAQ

What is the difference between naked and hidden in sudoku?

Naked patterns show directly in a cell’s own candidates; hidden patterns need checking unit by unit.

Are candidates and pencil marks the same thing?

Yes. Candidate is the term used here; pencil mark is the traditional paper-solving name for it.

What does “unit” mean in sudoku?

Unit is the general term for a row, column or box, the groups that each hold 1 to 9 once.

Ready to put a term into practice? Learn how to play or browse the full technique index.