The alpha helix (a-helix) is a common secondary structure of proteins and is a righthand-coiled or spiral conformation (helix) in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier ( hydrogen bonding). This secondary structure is also sometimes called a classic Pauling–Corey–Branson alpha helix (see below). The name 3.6₁₃-helix is also used for this type of helix, denoting the number of residues per helical turn, and 13 atoms being involved in the ring formed by the hydrogen bond. Among types of local structure in proteins, the a-helix is the most regular and the most predictable from sequence, as well as the most prevalent.