The cystine bond also known as the disulfide bond, sulfur bond, or just S bond is formed by cross-links between cystine residues (amino acids) of the main polypeptide chains. This bond is perpendicular to the axis of the hair and between the polypeptide chains. Because of its position in the hair, it is responsible for the hair's toughness or abrasion resistance. (It actually holds the hair fibers together.) These cross-links are frequent in the hair fiber, with maximum of frequency of one cystine bond every four turns of the alpha helix. This is what enables us to permanent wave the hair.