A) and B) illustrate parallel and anti-parallel dimeric coiled coils, respectively (images generated with coiled coils taken from PDB entries 2ZTA and 2NOV, respectively). Cylinders represent local orientation of helices, black spheres show Cα atoms and gray spheres depict Cβ atoms. Only a and d positions are shown. Given the direction of Cα-Cβ vectors on opposite chains, in anti-parallel alignments, a potential steric repulsion between a and a′ residues in adjacent layers (depicted with a dotted line) pushes the preference of ΔZoff away from zero.