The stability and the molecular order of nematic states of thermotropic polymers comprising rigid groups connected by flexible spacers are found to be dominated primarily by the characteristics (configurational partition function) of highly extended conformers, which are favoured for packing. Moreover, both the macroscopic consideration of the enthalpies and the entropies of isotropic-nematic transitions and the microscopic probe by deuterium n.m.r. of labelled chains lead to the conclusion that the highly extended conformers are selected preferentially in polymeric nematogens. A number of unique results exhibited by polymer liquid crystals, i.e. strong effects of even-odd character of polymethylene spacers, abnormalities associated with -C(=O)O- links [compared with -O- and -OC(=O)- links] between rigid (phenylene) groups and polymethylenes and drastic differences between polymethylene and polyoxyethylene spacers, can all be attributed quantitatively to this conformational ordering, which is the most prominent feature distinguishing polymer liquid crystals from their monomeric counterparts. The orientational order parameters of a nematic polymer measured from both D and H n.m.r. spectra of labelled chains are found to be quite high, ca. 0.8 throughout the nematic region. These findings on conformational order and orientational order of polymer liquid crystals are compared with theoretical predictions based on ideal lattice chains and worm-like chains.