A variety of techniques is used to describe the fine structure of a series of well defined linear polyethylenes, polymerized using the new generation of metallocene-based catalysts. The sample＇s molecular weight (M-w) is restricted to the usual values of processability of this material ranging from 14 K to 460 K. It will be clearly shown that the overall enthalpy of the rapidly crystallized samples follows a linear decrease with the log of M-w. The relative amounts of the different phases as seen from Raman spectroscopy and small angle X-ray scattering are evaluated as a function of molecular weight. As a consequence of the overall study, the contributions of the ordered and disordered regions to the long period are clearly separated and a semi-empirical expression for this essential structural parameter is proposed.