That linear chain to which all other chains, long or short or both,
may be regarded as being pendant.
Note: Where two or more chains could equally be considered to be the
main chain, that one is selected which leads to the simplest representation
of the molecule. 
In polymer science, the backbone chain or main chain of a polymer is the series of covalently bonded atoms that together create the continuous chain of the molecule.
In polypeptides, the backbone consists of carbon and nitrogen atoms of the constituent amino acids. The backbone does not include the side chains.