https://rostlab.org/papers/2008_rev_assignment2/paper.html#VORONO_TESSELLATION_FOR_GEOMETRICAL_RESIDUE_PARTITIONING
Converting secondary structure states to three classes

Several secondary structure assignment methods are presently available, but DSSP continues to be the most widely used method followed by STRIDE. In fact, most prediction methods are based on DSSP assignments. Typically, the 8 DSSP states are converted into three classes using the following convention: [GHI] -> h, [EB] -> e, [TS' '] -> c, which reads: 310-, a, p-helices are grouped into one helix class; Extended b-sheets and b-bridges are grouped into one sheet class; and the remaining secondary structure states turn, bend and Ňnot assignedÓ are grouped into one coil class.

Usually, 310-helices and b-bridges constitute short secondary structure segments that have some structural similarity to a-helix and b-strand, respectively. However, they do have different sequence characteristics. Prediction methods, in general, are more precise in the core of regular secondary structure segments than at the termini (Rost, 1994; Cuff, 1999). Thus, 310-helices and b-bridges are more difficult to predict than a-helices and b-strands. Therefore an alternative conversion that has been used more recently yields a seemingly higher level of prediction accuracy: [H] -> h, [E] -> e, [GIBTS' '] -> c.