The common twinning of tetrahedrite and tennantite can be described as an order-disorder (OD) phenomenon. The unit OD layer is a one-tetrahedron-thick (111) layer composed of six-member rings of tetrahedra, with gaps between them filled with Sb(As) coordination pyramids and triangular-coordinated (Cu, Ag). The stacking sequence of six-member rings is ABCABC, which can also be expressed as a sequence of three consecutive tetrahedron configurations, named alpha, beta, and gamma. When the orientation of component tetrahedra is uniform, the alpha, beta, gamma, alpha sequence builds the familiar cage structure of tetrahedrite. However, when the tetrahedra of the beta layer are rotated by 180 degrees against those in the underlying alpha configurations and/or when a rotated alpha configuration follows after the beta configuration (instead of gamma), twinning is generated. If repeated, this could generate the ABAB sequence which would modify the structure considerably. If the rest of the structure grows as a regular cubic tetrahedrite structure, the single occurrence of the described defect sequences creates a twin.