The Dirac equation describes free propagation of the spin particles. It has the form
Here is a spinor, describing the wave-function of a fermion with the mass , are gamma-matrices.

The Dirac equation is analogous to the Klein-Gordon equation for the scalar particles. Moreover, the property of gamma-matrices implies that the Dirac equation square is a Klein-Gordon (or wave) equation

The Dirac equation is analogous to the Klein-Gordon equation for the scalar particles. Moreover, the property of gamma-matrices implies that the Dirac equation square is a Klein-Gordon (or wave) equation