Excluding the end residues from the least-squares fit removes noise introduced from frayed ends, and more accurately classifies the helix type in the bulk of the structure. From the least-squares fit we report helix residues per turn for models with helical minimum energy clusters. From the least-square fit we report the root-mean squared error of the cylindrical fit, RM SE cyl , and the root-mean squared error of the helix fit, RM SE helix , as metrics of the quality of fit. All helix least-squares fits are detailed in the Supplemental Information section S3.