  We isolate and study the transformation of the intrinsic spin of Dirac
particles as they propagate along timelike geodesics in Kerr geometry.
Reference frames play a crucial role in the definition and measurement of the
intrinsic spin of test particles. We show how observers located in the outer
geometry of Kerr black holes may exploit the symmetries of the geometry to set
up reference frames using purely geometric, locally-available information.
Armed with these geometrically-defined reference frames, we obtain a
closed-form expression for the geometrically-induced spin precession of Dirac
particles in the outer geometry of Kerr black holes. We show that the spin of
Dirac particles does not precess on the equatorial place of Kerr geometry; and
hence, in Schwarschild geometry.
