A quasi-binary (QB) picture of thermotropic liquid crystals is proposed on the basis of thermodynamic observations. The experimental conformational entropy of long alkyl chains attached to a (semi)rigid core of mesogenic molecules indicates that the chain is highly disordered in liquid crystalline states. These disordered chains serve as 'intramolecular solvent' or 'self-solvent' judging from a close resemblance between phase diagrams of neat (against chain length) and binary (against composition) systems. The application of the QB picture to the classic examples of thermotropic cubic mesophases (in ANBC series) shows that the essential structural motif is triply periodic minimal surface. (C) 2002 Elsevier Science B.V. All rights reserved.