Many proteins may adopt a beta sheet as part of their secondary structure. In beta sheets, sections of a single polypeptide may run side-by-side and antiparallel to each other, to allow for hydrogen bonding between their backbone chains. Beta sheets can also be either a parallel or anti-parallel secondary structure. However, an anti-parallel beta sheet is significantly more stable than a parallel structure due to their well aligned H-bonds, which are at a 90° angle.