Examples of surface junction images that could not be aligned. A and B are representative images in different cells. Yellow dots were placed by an experienced observer to mark possible clusters of Ca_Vs attached to an underlying RYR. The grid representing the putative RYR array was scaled and rotated to align with the clusters marked by arrows, deemed to be in canonical configuration (i.e., constituting well configured tetrads, oriented parallel to one or more neighbors). In all cases, most of the other identified clusters were not aligned with the grid, due to rotation or spatial shift. Images like these were excluded from the numerical averages. Tetrads are ∼30 nm on each side.