Digon

A digon is a polygon with two sides. It is degenerate if embedded in Euclidean space, as its edges coincide. It can however be thought of as a tiling of the circle. In two-dimensional or higher spherical space, it can form a lune such as the ones making up a hosohedron.

It is the only two-dimensional ditope and hosotope. It is also the two-dimensional demihypercube.