  The propagator of a gauge boson, like the massless photon or the massive
vector bosons $W^\pm$ and $Z$ of the electroweak theory, can be derived in two
different ways, namely via Green's functions (semi-classical approach) or via
the vacuum expectation value of the time-ordered product of the field operators
(field theoretical approach). Comparing the semi-classical with the field
theoretical approach, the central tensorial object can be defined as the gauge
boson projector, directly related to the completeness relation for the complete
set of polarisation four-vectors. In this paper we explain the relation for
this projector to different cases of the $R_\xi$ gauge and explain why the
unitary gauge is the default gauge for massive gauge bosons.
