Sudoku Glossary

Every term you'll meet in guides, forums, and this site — in plain English

Sudoku writing is full of jargon — naked this, hidden that, fish everywhere. Here's the vocabulary in one place, organised from the board up. Terms in bold have their own entry.

The board

• Cell — one of the 81 squares in the grid. Each holds a single digit 1–9 when solved.
• Row / Column — the nine horizontal / vertical lines of cells. Each must contain 1–9 exactly once.
• Box (also block, region, nonet) — one of the nine 3×3 squares. Same rule: 1–9, once each.
• Group (also house, unit) — any row, column, or box; the 27 constraints of the puzzle.
• Clue (also given) — a digit printed in the grid at the start. The proven minimum for a unique classic Sudoku is 17 clues.
• Peer — every cell sharing a group with a given cell. Each cell has 20 peers, and no peer may repeat its digit.

Working the puzzle

• Candidate (also pencil mark, note) — a small digit noted in a cell meaning "this could still go here." Managing candidates honestly is the foundation of every technique past beginner level.
• Scanning — sweeping a digit across the grid, using existing placements to rule out rows and columns, to find where it must land.
• Naked single — a cell with exactly one candidate left. Place it.
• Hidden single — a digit with exactly one possible home in a group, even though that cell shows other candidates. Place it.
• Locked candidates / pointing pair — a digit's candidates in a box confined to one row or column, letting you erase it along the rest of that line (and vice versa: box-line reduction).
• Naked pair / triple — two (three) cells in a group holding only the same two (three) candidates between them; erase those digits elsewhere in the group. See our full pairs guide.
• Hidden pair / triple — two (three) digits confined to the same two (three) cells of a group; erase everything else from those cells.

Advanced patterns

• Fish — the family of single-digit patterns spanning multiple rows and columns.
• X-Wing — a digit locked into the four corners of a rectangle across two rows and two columns; eliminates along the crossing lines. The 2×2 fish — see the X-Wing & Swordfish guide.
• Swordfish — the 3×3 fish: three rows confined to three shared columns (or the reverse).
• Jellyfish — the 4×4 fish. Legal, spectacular, and almost never required by a well-made puzzle.
• XY-Wing (also Y-Wing) — three two-candidate cells linked in a pivot-and-pincers pattern; whatever the pivot turns out to be, both pincers force the same elimination.
• Chain / forcing chain — a linked sequence of if-then deductions across the grid. The general engine behind most expert techniques.

Puzzle properties

• Unique solution — the defining promise of a proper Sudoku: exactly one valid completion. Every one of Cosmic Sudoku's 777 levels is verified unique and solvable by logic alone.
• Difficulty rating — a grade based on the hardest technique a solve requires, not on clue count — explained in difficulty explained.
• Symmetry — the tradition (from Japanese publisher Nikoli) that clue positions form a rotationally symmetric pattern.
• Minimal puzzle — a puzzle where removing any single clue destroys uniqueness.
• Variant — any rule-twist on the classic: killer (cage sums), diagonal/Sudoku X, samurai (five overlapping grids), 6×6 minis, and dozens more.

Etiquette of the craft

• Guessing (also bifurcation) — picking a candidate and seeing what breaks. Works, but teaches nothing; a well-made puzzle never requires it.
• Streak — consecutive days solving the daily puzzle. Guard it jealously.

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