- ➊ Blacken some cells of the diagram [paper] resp. color the gray cells of the diagram black or white [interactively].
- ➋ Orthogonally contiguous black cells form a group. Each region contains exactly two groups of black cells which must be congruent (i.e. the two groups must have the same size and shape, or one must have the reflected shape of the other).
- ➌ A cell with a number is always white. The arrow points to an adjacent cell belonging to one of the two black groups in the area. The number indicates the size of this group.
- ➍ Black cells must not be orthogonally adjacent across region borders.