The beta prism is a fold with three internal repeats, each being an antiparallel beta sheet with (+1, +1, -3) connectivity. The three sheets are arranged like the faces of a triangular prism, with internal pseudo-threefold symmetry (Shimizu and Morikawa, 1996). There are two types of prism: the orthogonal prism, in which the strands are perpendicular to the prism axis; and the aligned prism, in which the strands are parallel to the prism axis.