  We present here the results of the room temperature dielectric constant
measurements and Rietveld analysis of the powder x-ray diffraction data on
$(1-x)[Pb(Mg_{1/3}Nb_{2/3})O_3]-xPbTiO_3$(PMN-$x$PT) in the composition range
$0.20 \leq x \leq 0.45$ to show that the morphotropic phase boundary (MPB)
region contains two monoclinic phases with space groups Cm (or $M_B$ type) and
Pm (or $M_C$ type) stable in the composition ranges $0.27 \leq x \leq 0.30$ and
$0.31 \leq x \leq 0.34$, respectively. The structure of PMN-$x$PT in the
composition ranges $0 \leq x \leq$ 0.26, and $0.35 \leq x \leq1$ is found to be
rhombohedral (R3m) and tetragonal (P4mm), respectively. These results are
compared with the predictions of Vanderbilt & Cohen's theory.
