<p>Beta barrels are commonly observed in protein structures. They are classified in terms of two integral parameters: the number of strands in the sheet, n, and the shear number, S, a measure of the stagger of the strands in the beta-sheet. These two parameters have been shown to determine the major geometrical features of beta-barrels. Six-stranded beta-barrels with a pseudo-twofold axis are found in several proteins. One involving parallel strands forming two psi structures is known as the double-psi barrel. The first psi structure consists of the loop connecting strands beta1 and beta2 (a 'psi loop') and the strand beta5, whereas the second psi structure consists of the loop connecting strands beta4 and beta5 and the strand beta2. All the psi structures in double-psi barrels have a unique handedness, in that beta1 (beta4), beta2 (beta5) and the loop following beta5 (beta2) form a right-handed helix. The unique handedness may be related to the fact that the twisting angle between the parallel pair of strands is always larger than that between the antiparallel pair [<cite idref="PUB00011777"/>].</p><p>In many cases, including aspartate decarboxylase and aspartic proteinases, strands 1 and 4 are each bent and consist of two sections. The two sections normally make a right angle; sometimes their hydrogen-bond patterns are disrupted at the corner by a bulge or even by a large insertion. In these cases, the barrel can also be viewed as a pair of orthogonally packed sheets, each with four strands. </p>