Beta bulges are regions of irregularity in a beta sheet, where the normal pattern of hydrogen bonding is disrupted, e.g. by the insertion of an extra residue. A bulge usually involves two or more residues in the bulged strand, opposite a single residue in the adjacent strand. These irregularities have been identified and classified automatically. In all, five types of beta-bulge were identified: classic, G1, wide, bent and special. Bulges were found to occur frequently in proteins, on average more than two bulges per protein. In general, beta-bulges produce two main changes in the structure of a beta sheet: (i)disruption of the normal alternation of side-chain direction (ii) accentuation of the twist of the sheet, altering the direction of the surrounding strands.