Secondary structure refers to parts of the protein chain that form regular structures. There are two main types of secondary structure: alpha helix and beta strand (beta sheet). Alpha helices are coils that turn in a clockwise rotation and beta strands form a beta sheet where the chains lie alongside each other. The two types of secondary structure were suggested in 1951 by Linus Pauling and coworkers. Unlike the polypeptide chain, which is bonded through covalent peptide bonds, secondary structure is formed by hydrogen bonds.