  Our understanding of the three basic states of matter (solids, liquids and
gases) is based on temperature and pressure phase diagrams with three phase
transition lines: solid-gas, liquid-gas and solid-liquid lines. There are
analytical expressions $P(T)$ for the first two lines derived on a purely
general-theoretical thermodynamic basis. In contrast, there exists no similar
function for the third, melting, line (ML). Here, we develop a general
two-phase theory of MLs and their analytical form. This theory predicts the
parabolic form of the MLs for normal melting, relates the MLs to thermal and
elastic properties of liquid and solid phases and quantitatively agrees with
experimental MLs in different system types. We show that the parameters of the
ML parabola are governed by fundamental physical constants. In this sense,
parabolic MLs possess universality across different systems.
