A classification according to the content of α-helix and β-pleated sheet. A protein is designated <α if it has a minimum number of residues in an α-helix and fewer than a maximum in β-pleated sheets (suggested to be 45% and 5%, respectively); it is α, if it has less than a maximum of α-helix and more than a minimum of β-sheet (5% and 40%, respectively); αβ, if it has minimum of both α-helix and anti-parallel β-sheet (15% and 15%); and α/β, if it has a minimum of both α-helix and parallel β-sheet. It is irregular (ξ) if it has less than 5% α-helix and 10% β-sheet.