The discontinuities found in heptad repeats of α‐helical coiled‐coil proteins have been characterized. A survey of 40 α‐fibrous proteins reveals that only two classes of heptad breaks are prevalent: the stutter, corresponding to a deletion of three residues, and the newly identified “stammer,” corresponding to a deletion of four residues. This restriction on the variety of insertions/deletions encountered gives support to a unifying structural model, where different degrees of supercoiling accommodate the observed breaks. Stutters in the hemagglutinin coiled‐coil region have previously been shown to produce an underwinding of the supercoil, and we show here how, in other cases, stammers would lead to overwinding. An analysis of main‐chain structure also indicates that the mannose‐binding protein, as well as hemagglutinin, contains an underwound coiled‐coil region. In contrast to knobs‐into‐holes packing, these models give rise to non‐close‐packed cores at the sites of the heptad phase shifts. We suggest that such non‐close‐packed cores may function to terminate certain coiled‐coil regions, and may also account for the flexibility observed in such long α‐fibrous molecules as myosin. The local underwinding or overwinding caused by these specific breaks in the heptad repeat has a global effect on the structure and can modify both the assembly of the protein and its interaction properties. © 1996 Wiley‐Liss, Inc.