In a protein chain three dihedral angles are defined as "φ" (phi), "ψ" (psi) and "ω" (omega), as shown in the diagram. The planarity of the peptide bond usually restricts "ω" to be 180° (the typical "trans" case) or 0° (the rare "cis" case). The distance between the Cα atoms in the "trans" and "cis" isomers is approximately 3.8 and 2.9 Å, respectively. The vast majority of the peptide bonds in proteins are "trans", though the peptide bond to the nitrogen of proline has an increased prevalence of "cis" compared to other amino-acid pairs.