Components of a junction between an SR terminal cisterna and a transverse (T) tubule. (A) Freeze-dried junctional SR membrane from guinea pig showing RyR1 tetramers of ∼28-nm sides. (B) Tetrads of particles (Ca_Vs) in a freeze-fractured T tubule membrane from toadfish muscle, presented with the same orientation and magnification. (C) Canonical couplon in which every other channel is in contact with Ca_Vs (orange elements). The stoichiometric overlap of Ca_Vs and RyRs, first proposed by Block et al. (1988), was confirmed by direct observation by Xu et al. (2024); see Fig. 7 below. The diagram illustrates chirality or handedness, as viewed from outside the cell, in the way RyR tetramers make mutual contact. This orientation, conventionally called right-handed, has the intertetramer approaches occurring at the tetramers’ edge, to the right-hand side of every corner. The contacts are said to be antiparallel, as the corners of the tetramers involved are on opposite ends of the contact segment. The overlapping Ca_Vs make the contact asymmetric, a feature relevant for the dynamics of the interactions. Relabeled Fig. 1 of Ríos et al. (2019), which includes images from Block et al. (1988).