We plotted the structures of some typical interface fragment pairs that are the most abundant in the library. The two most abundant clusters are shown in (a) and (b), both of which are HH motif. The direction of each α-helix is indicated by the arrow in the figures. Moreover, the interface fragment pairs from the SS motif are shown in (c), (d) and (e). Two fragments are either from a same piece of β-sheet that are connected by hydrogen bonds, or from two β-sheets facing each other. Finally, fragment pairs involving loops are shown in (f), (g) and (h).