The β sheet (also β-pleate sheet) is the second form of regular secondary structure in proteins consisting of beta strands connected laterally by three or more hydrogen bonds, forming a generally twisted, pleated sheet (the most common form of regular secondary structure in proteins is the alpha helix). A beta strand (also β-strand) is a stretch of amino acids typically 5–10 amino acids long whose peptide backbones are almost fully extended. The association of beta sheets has been implicated in the formation of protein aggregates and fibrils observed in many human diseases, notably the amyloidoses.