Components of a triadic junction visible in EM images. (A) Freeze-dried rotary shadowed junctional SR membrane from guinea pig. (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, with array notation (side view in side diagram). RYR tetramers (or channels, or feet; green) are identified by a row index j (which in T tubule couplons range between 0 and 1) and a column index k (0–3 in the case illustrated). Whether channels are even or odd is determined by the parity of j + k. In this canonical configuration, even channels are fully occupied by Ca_Vs (orange elements). Individual protomers are identified by index i (0–3), increasing clockwise within the RYR tetramer. The adjacent feet of foot (0, 0) are (0, 1) and (1, 0). In those, the adjacent protomers are (0, 0, 1), adjacent to (2, 0, 0), and (1, 1, 0), adjacent to (3, 0, 0). (D) Diagram to illustrate chirality or handedness in the couplon in the conventional view, which results from viewing junctions from outside the cell. We arbitrarily designate this orientation as right handed. The horizontal distance between centers of adjacent RYRs is ∼30 nm.