Motifs
Motifs were discovered after many 3D structures were available for examination. A motif is a simple geometric arrangement of one or more secondary structural elements -- for example helix-turn-helix (see image) used in DNA binding -- that occurs in many different proteins. It falls somewhere between secondary and tertiary structure in terms of level of organizaiton. Unlike domains, motifs are incapable of independent folding. A domain may contain zero, one, two, etc motifs within its structure.