The major coat protein of the filamentous bacteriophage M13 is a surprising protein because it exists both as a membrane protein and as part of the M13 phage coat during its life cycle. Early studies showed that the phage-bound structure of the coat protein was a continuous I-shaped α-helix. However, throughout the years various structural models, both I-shaped and L-shaped, have been proposed for the membrane-bound state of the coat protein. Recently, site-directed labelling approaches have enabled the study of the coat protein under conditions that more closely mimic the in vivo membrane-bound state. Interestingly, the structure that has emerged from this work is I-shaped and similar to the structure in the phage-bound state.