A motif of three residues within a beta-sheet in which the main chains of two consecutive residues are H-bonded to that of the third, and in which the dihedral angles are as follows: Residue(i): -140 degrees < phi(l) -20 degrees , -90 degrees < psi(l) < 40 degrees. Residue (i+1): -180 degrees < phi < -25 degrees or +120 degrees < phi < +180 degrees, +40 degrees < psi < +180 degrees or -180 degrees < psi < -120 degrees.