- ➊ Blacken some cells of the diagram [paper] resp. color the
gray cells of the diagram black or white [interactively].
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