X-ray diffraction pattern of relaxed, permeabilized plaice muscle. Half-mirrored pattern shows equator (Eq), meridian (Mer), and main myosin layer lines (M1–M6). Numerals 10, 11, 20, and 21 indicate sampling of M1 by equatorial reflections 10, 11, 20, and 21. Beam stop is shifted slightly off center to expose sampling of 10 reflection on right side of M1 layer line.