Peptide plane flipping is a type of conformational change that can occur in proteins by which the dihedral angles of adjacent amino acids undergo large-scale rotations with little displacement of the side chains. The plane flip is defined as a rotation of the dihedral angles φ,ψ at amino acids i and i+1 such that the resulting angles remain in structurally stable regions of Ramachandran space.  The key requirement is that the sum of the ψ angle of residue i and the φ angle of residue i+1 remain roughly constant; in effect, the flip is a crankshaft move about the axis defined by the C-C¹ and N-C bond vectors of the peptide group, which are roughly parallel.  As an example, the type I and type II beta turns differ by a simple flip of the central peptide group of the turn.