The polypeptide chain. The sequence of R-groups along the chain is called the primary structure.
Secondary structure refers to the local folding of the polypeptide chain.
Tertiary structure is the arrangement of secondary structure elements in 3 dimensions and quaternary structure describes the arrangement of a protein's subunits. Linus Pauling and Robert Corey analysed the geometry and dimensions of the peptide bonds in the crystal structures of molecules containing one or a few peptide bonds. Their results are summarised in this diagram where the consensus bond lengths are shown in Angstrom units. Bond angles in degrees are also shown for the peptide N and C atoms. This rigidity of the peptide bond reduces the degrees of freedom of the polypeptide during folding.