A Ramachandran plot (also known as a Ramachandran diagram or a [φ,ψ] plot), originally developed in 1963 by G. N. Ramachandran, C. Ramakrishnan, and V. Sasisekharan, is a way to visualize energetically allowed regions for backbone dihedral angles ψ against φ of amino acid residues in protein structure.
In a protein chain three dihedral angles are defined:
* ω (omega) is the angle in the chain C − C' − N − C,
* φ (phi) is the angle in the chain C − N − C − C
* ψ (psi) is the angle in the chain N − C − C − N (called φ′' by Ramachandran)
The figure at right illustrates the location of each of these angles (but it does not show correctly the way they are defined).
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 C 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.
The side chain dihedral angles are designated with χ (chi-n). They tend to cluster near 180°, 60°, and −60°, which are called the trans, gauche, and gauche conformations.  The stability of certain sidechain dihedral angles is affected by the values φ and ψ. For instance, there are direct steric interactions between the Cγ of the side chain in the gauche rotamer and the backbone nitrogen of the next residue when ψ is near -60°. This is evident from statistical distributions in backbone-dependent rotamer libraries.