  Spin-relaxation is conventionally discussed using two different approaches
for materials with and without inversion symmetry. The former is known as the
Elliott-Yafet (EY) theory and for the latter the D'yakonov-Perel' (DP) theory
applies, respectively. We discuss herein a simple and intuitive approach to
demonstrate that the two seemingly disparate mechanisms are closely related. A
compelling analogy between the respective Hamiltonian is presented and that the
usual derivation of spin-relaxation times, in the respective frameworks of the
two theories, can be performed. The result also allows to obtain the less
canonical spin-relaxation regimes; the generalization of the EY when the
material has a large quasiparticle broadening and the DP mechanism in ultrapure
semiconductors. The method also allows a practical and intuitive numerical
implementation of the spin-relaxation calculation, which is demonstrated for
MgB$_2$ that has anomalous spin-relaxation properties.
