Thus, two types of polytypes were found: ABAB in both chalcogenides and AB 1 CD 1 AB 1 CD 1 in B 6 Se. Polytypism is defined as the ability of a compound to crystallize in several structural modifications, each of which can be considered as built up by stacking layers of (almost) identical structure; the modifications differing only in their stacking sequence. Polytypes have two common unit cell parameters, and the third one is a variable integer multiple of the same value equal to the spacing between the nearest neighboring layers . In this context, polymorphism is sometimes called one-dimensional polymorphism.