This paper concerns an empirical linear relationship between the melting temperatureT1and the second‐order or glassy transition temperatureT2for partially crystalline polymers. Two lines can be drawn, one for symmetrical polymers, the other for unsymmetrical polymers, the latter being of lower slope. These families are only approximate. It is shown for a limited group of polymers, thatT2is proportional to the cohesive energy density of the polymer. SinceT1=&Dgr;H/&Dgr;S, it is suggested that the two families of curves arise because of differences in &Dgr;Swhich depends on symmetry and chain stiffness. It is shown thatT1andT2are linearly related with diluent as a common parameter.