  We introduce new tetrads that manifestly and covariantly diagonalize the
stress-energy tensor for a perfect fluid with vorticity at every spacetime
point. We discuss the origin of inertia in this special case from the
standpoint of our new local tetrads. We believe that this new tetrad might
bring about simplification in the analysis of astrophysical relativistic
problems where vorticity is present, for instance through the
Carter-Lichnerowicz equation.
