When amino acids combine they do so via the formation of an amide bond between the alpha-carboxyl of one amino acid and the alpha-amine of the other. This amide bond is termed a peptide bond. We can visialize this peptide bond using the dipeptide Ala-Ser.

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The peptide bond has two resonance forms which makes the bond have double bond character. Due to this double bond character rotation about the peptide bond is restricted. This in turn restricts the possible motions of the peptide backbone. In addition to the restriction of rotation about the peptide bond, its double bond character also places six atoms of the peptide group in the same plane. This plane is called the peptide plane.

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