The fundamental structural unit, the right-handed triple helix, consists of three coiled subunits, chains that are wound around each other. These chains are unusually rich in glycine, proline, and hydroxyproline, which allow them to form the triple helix -- the side chain of glycine (an H atom) is the only one small enough to fit into the center of a three-stranded helix. The rigid triple helix geometry is fixed by the angle of the C-N bond of peptidyl-proline or peptidyl-hydroxyproline. The chains subunits are held together by hydrogen bonds that link the peptide amine bonds of glycine residues to peptide carbonyl groups in an adjacent polypeptide. The triple helix of collagen is 300 nm long, and only 1.5 nm wide.