- ➊ A domino consists of two half-Dominoes. Each half-domino shows a number between 0 and N.
- ➋ Each domino exists exactly once: 0-0, 0-1, etc. up to N-N. Note that there is no difference between X-Y and X-Y (e.g. 1-2 and 2-1).
- ➌ Divide the diagram along the grid lines into dominoes so that each domino occurs exactly once.