Boundary word

The word formed by starting on a point of the perimeter of a polyomino and tracking the heading, adding a new letter each time the polyomino turns (for example, North, East, South, West).

By convention, the boundary word starts at the left-most point on the bottom row of the polyomino and proceeds counter-clockwise.

For example, the following polyomino:

has the boundary word ENWSWS.

Polyominoes with without punctures have only one boundary word, but punctured polyominoes have additional boundary words for each puncture.

Polyominoes that share the same boundary word belong in the same class.