A predictive rule for protein folding is presented that involves two recurrent glycine-based motifs that cap the carboxyl termini of α helices. In proteins, helices that terminated in glycine residues were found predominantly in one of these two motifs. These glycine structures had a characteristic pattern of polar and apolar residues. Visual inspection of known helical sequences was sufficient to distinguish the two motifs from each other and from internal glycines that fail to terminate helices. These glycine motifs—in which the local sequence selects between available structures—represent an example of a stereochemical rule for protein folding.