Each fragment was divided into three categories, depending on the secondary structure type of the residue at the center of the fragment. Consequently, the interface fragment pair was classified into six motifs. The percentage of these six motifs for all the 459 fragment pairs in the library is plotted in (a). We further defined a preference score for each motif. A higher score for a specific motif indicates that it is more favored to form. Consequently, the preference scores for all six motifs are plotted in (b). The figure suggests that fragment pair motifs are not equally distributed, but have strong preference.