Length requirements for the ideal polyalanine α-helix search model. Helices between five and 14 residues were used as models, and one to eight copies were searched for in PHASER, followed by three cycles of density modification and automated polyalanine chain tracing in SHELXE. All calculations were done at the maximum resolution of 1.82 Å. The solvent content was set to 0.65, slightly higher than expected in order to help solvent flattening, and in the auto-tracing α-helices were searched for in all cycles (-q option). The NCS option in SHELXE is available for substructures but not for tracing and hence was not applicable here. Helices between eight and 11 residues are suitable search models as judged by CC > 25%. The most extensive search performed was for nine fragments of a decamer helix, which yielded a final LLG of 576. For improved legibility the data points (black spheres) are projected onto the grey walls of the plot (blue and white dots).