Rules & terminology

When a digit has exactly two candidate cells in a unit. The underlying configuration that strong links, X-wings, and chain techniques all reach for as their starting shape.

In Killer Sudoku, a contiguous group of cells outlined by a dotted line, with a printed sum the digits inside must add up to. Replaces the classic Sudoku given.

A digit (1–9) a cell could still legally hold — one not yet ruled out by anything in its row, column, or 3×3 box. Every empty cell has between one and nine.

The dark cell in a kakuro grid carrying the target sums for the runs extending from it — usually one or two numbers separated by a slash.

A white cell in a kakuro grid that the player fills with a digit. Each entry cell belongs to exactly two runs — one horizontal, one vertical — and must satisfy both.

A digit pre-filled into a cell at puzzle start — a clue placed by the puzzle's setter. Givens cannot be changed by the player; the rest of the grid has to be solved around them.

A Japanese number-placement puzzle. Cross-sums act like a crossword grid; players fill white cells with digits 1–9 so each run sums to its target without repeating a digit.

A small handwritten or app-rendered note inside a cell indicating which digits the cell could still legally hold. The visible representation of a cell's candidate set.

A horizontal or vertical sum-segment in a kakuro grid. The white cells from one clue cell to the next, which must sum to the clue's target without repeating a digit.

A relationship between two cells in a unit where a digit must occupy exactly one of them — the basic primitive that hidden singles, X-wings, and chain reasoning all rest on.

Collective name for a row, column, or 3×3 box — the three groupings Sudoku's no-repeats rule applies to. Every cell sits in exactly three: its row, column, and box.

A relationship between two cells where at most one can hold the digit. Looser than a strong link — both might be other digits — and the steady half of every chain technique.