  We present a set of once subtracted dispersion relations which implement
crossing symmetry conditions for the $\pi\pi$ scattering amplitudes below 1
GeV. We compare and discuss the results obtained for the once and twice
subtracted dispersion relations, known as Roy's equations, for three $\pi\pi$
partial JI waves, S0, P and S2. We also show that once subtracted dispersion
relations provide a stringent test of crossing and analyticity for $\pi\pi$
partial wave amplitudes, remarkably precise in the 400 to 1.1 GeV region, where
the resulting uncertainties are significantly smaller than those coming from
standard Roy's equations, given the same input.
