  We present a simple and flexible method of keeping track of the complex
phases and spin quantization axes for half-spin initial- and final-state Weyl
spinors in scattering amplitudes of Standard Model high energy physics
processes. Both cases of massless and massive spinors are discussed. The method
is demonstrated and checked numerically for spin correlations in tau tau-bar
production and decay. Its main application will be in the forthcoming work of
combining effects due to multiple photon emission (exponentiation) and spin,
embodied in the Monte Carlo event generators for production and decay of
unstable fermions such as the tau lepton, t-quark and hypothetical new heavy
particles.
