A peptide region which hydrogen bonded to another region of peptide running in the oposite direction (one running N-terminal to C-terminal and one running C-terminal to N-terminal). Hydrogen bonding occurs between every other C=O from one strand to every other N-H on the adjacent strand. In this case, if two atoms C-alpha (i) and C-alpha (j) are adjacent in two hydrogen-bonded beta strands, then they form two mutual backbone hydrogen bonds to each other's flanking peptide groups; this is known as a close pair of hydrogen bonds. The peptide backbone dihedral angles (phi, psi) are about (-140 degrees, 135 degrees) in antiparallel sheets.