  The effect of the relativistic spin rotation, conditioned by the setting of
the spin in the rest frame of a particle and by the noncommutativity of the
Lorentz transformations along noncolinear directions, is discussed. In
connection with this, the Thomas precession of the spin polarization vector at
the motion of a particle along a curvilinear trajectory is considered. The
transformations of the correlation tensor components for a system of two
spin-1/2 particles at the transition from the c.m.s. of the particle pair to
the laboratory frame are investigated. When the particle laboratory velocities
are not colinear, the relativistic spin rotation angles for these particles are
different. As a result, the relative fractions of the singlet and triplet
states in the relativistic system of two free spin-1/2 particles with a nonzero
vector of relative momentum depend on the concrete frame in which this
two-particle system is analyzed.
