CageDokuSudoku and Killer Sudoku Glossary
By Andrew Boyko, developer of CageDoku · Updated
Short definitions of the terms you meet in Killer Sudoku, classic Sudoku and the variants in CageDoku.
- Cage
- A group of cells outlined by a dotted line in Killer Sudoku. The small number in its corner is the sum of its digits, and digits cannot repeat inside a cage.
- Cage sum
- The total of the digits in a cage, printed in its top-left corner.
- Candidate
- A digit that could still go in a cell. Players write them as small “pencil marks”.
- Combination
- A set of different digits that adds up to a cage sum, such as 1-2-5 or 1-3-4 for a 3-cell cage with sum 8.
- Unique combination
- A cage sum with only one possible set of digits, for example two cells summing to 17 (8 and 9).
- 45 rule
- Every row, column and 3×3 box contains 1 to 9 once, so it sums to 45. Used to deduce cells from cage sums.
- Innie
- A cell inside a region (row, column or box) whose cage extends outside the region.
- Outie
- A cell outside a region that belongs to a cage reaching into the region.
- Region (unit)
- A row, column or box: any group of nine cells that must contain 1 to 9 once.
- Naked single
- A cell with only one remaining candidate.
- A digit that fits in only one cell of a region, even if that cell has other candidates.
- Naked pair
- Two cells in a region that contain the same two candidates, which locks those digits and removes them from the other cells of the region.
- Two digits that can only go in the same two cells of a region, so those cells can drop all other candidates.
- Locked candidates
- A digit confined to one line inside a box (or to one box inside a line), which removes it from the rest of the line or box.
- X-Wing
- A pattern in which a digit appears in exactly two places in each of two rows and those places share two columns, allowing eliminations elsewhere in those columns.
- Swordfish
- The three-row, three-column version of the X-Wing.
- Thermometer (Thermo)
- A path of cells in Thermo Sudoku, starting at a round bulb. Digits strictly increase from the bulb to the tip.
- Arrow
- A path of cells in Arrow Sudoku whose digits add up to the digit in the circle at its start.
- Kropki dot
- A dot between two cells in Kropki Sudoku: white means consecutive digits, black means one digit is double the other.
- Sandwich clue
- A number outside the grid in Sandwich Sudoku: the sum of the digits between the 1 and the 9 in that row or column.
- Anti-knight (Chess) rule
- A rule that forbids the same digit in two cells a chess knight’s move apart.
- Sudoku X (Diagonal)
- A variant where both main diagonals must also contain 1 to 9 once.
- Given
- A digit printed in the puzzle from the start.
- Complement
- For big cages, the set of digits left out. A 7-cell cage with sum 40 leaves out two digits adding to 5.
Practice it in CageDoku
Free, ad-free Killer Sudoku and seven more variants. Hints name the technique and highlight the cells, so you learn why a digit goes where it goes.