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.
Source:
PAC, 1996, 68, 2287. 'Glossary of basic terms in polymer science (IUPAC Recommendations 1996)' on page 2294 (https://doi.org/10.1351/pac199668122287)