Divide the entire number grid into domino tiles. For example, a 4×5 grid must be split into exactly 10 dominoes: [0|0], [0|1], [0|2], [0|3], [1|1], [1|2], [1|3], [2|2], [2|3], [3|3].
Rules
A domino pairs two horizontally or vertically adjacent cells. For example, if cells show '2' and '5' side by side, they form a [2|5] domino.
Each number pair appears exactly once. If you've already placed a [1|3] domino, no other pair of cells can form another [1|3].
Every cell must belong to exactly one domino — no gaps and no overlaps allowed.
Mark edges with X to eliminate possibilities. Turn on X-mark mode and select two adjacent cells to mark 'these are NOT a domino pair'.
Key strategy: If a number pair (e.g. [0|3]) appears adjacent in only one place on the grid, that domino's position is certain. Find these 'forced' dominoes first.
Use the reference grid at the bottom (mobile) or side (desktop). Dominoes without a ✓ mark haven't been placed yet.
Dominosa puzzles train combinatorial reasoning. Systematic elimination is a core deductive reasoning skill linked to prefrontal cortex activation.
Goel, V. & Dolan, R.J. (2004). Differential involvement of left prefrontal cortex in inductive and deductive reasoning. Cognition, 93(3), B109-B121.
Ferreira, N., et al. (2015). Sudoku — a brain-teaser or a mental health aid? PLOS ONE, 10(3), e0120548.