Connectivity of the sheath subunits in the extended (A) and contracted (B) tail sheath reprinted from . The cryoEM map of the entire tail is shown on the far left. Immediately next to it, the three adjacent helices (in pink, blue and green) are shown to permit a better view of the internal arrangement. The successive hexameric discs are numbered 1, 2, 3, 4 and 5 with disc number 1 being closest to the baseplate. In the middle panels are the three helices formed by domains I, II and III. On the right is the arrangement of domain IV, for which the crystal structure is unknown. This domain retains the connectivity between neighboring subunits within each helix in both conformations of the sheath. C, One sixth of the gp18 helix - one strand - is shown for the extended (green) and contracted (golden brown) sheath conformations.