This is a diagram of the sterically allowed regions for a dipeptide unit in phi,psi space. The conformation of the dipepetide is defined by 2 dihedral angles: phi (the angle of rotation around the N-C-alpha bond in a peptide unit), and psi (the angle of rotation around the C-alpha-C' bond in a peptide unit). The plot indicates whether a given phi,psi conformation is sterically allowed. A conformation is not allowed if atoms approach closer than the sum of their van der Waals radii. The diagram is named after G.N Ramachandran, who was the first to make calculations of the allowed regions in protein structures.