A form of secondary structure of a protein in which the amide hydrogens of a peptide bond of one extended polypeptide sequence are shared with the carbonyl oxygens of a peptide bond on a second polypeptide sequence. A sheet that often consists of three or more polypeptide sequences is said to be parallel (i.e. both adjacent strands run in the same direction; N- to C-terminal) or antiparallel. see also protein class