The study investigates the high piezoelectric response in PbZr$_{1-x}$Ti$_x$O$_3$ (PZT) ceramics near the morphotropic phase boundary (MPB). High-resolution X-ray powder diffraction measurements reveal that the piezoelectric elongation of the unit cell occurs along directions associated with monoclinic distortion, rather than along polar directions. This finding provides direct evidence for the origin of the high piezoelectricity in PZT. The research highlights the role of the recently discovered stable monoclinic phase in explaining the enhanced piezoelectric response in PZT and similar systems. The study uses samples with compositions x=0.42, 0.45, and 0.48, prepared by conventional solid-state reaction techniques, and characterized using high-resolution synchrotron X-ray diffraction. The results show that poling induces significant changes in the unit cell dimensions along the [001] direction in rhombohedral PZT and along [101] in tetragonal PZT, confirming the predicted behavior. The findings support a model based on local monoclinic shifts superimposed on rhombohedral and tetragonal displacements, and recent first-principles calculations have successfully reproduced the monoclinic phase and explained the high piezoelectric coefficients.The study investigates the high piezoelectric response in PbZr$_{1-x}$Ti$_x$O$_3$ (PZT) ceramics near the morphotropic phase boundary (MPB). High-resolution X-ray powder diffraction measurements reveal that the piezoelectric elongation of the unit cell occurs along directions associated with monoclinic distortion, rather than along polar directions. This finding provides direct evidence for the origin of the high piezoelectricity in PZT. The research highlights the role of the recently discovered stable monoclinic phase in explaining the enhanced piezoelectric response in PZT and similar systems. The study uses samples with compositions x=0.42, 0.45, and 0.48, prepared by conventional solid-state reaction techniques, and characterized using high-resolution synchrotron X-ray diffraction. The results show that poling induces significant changes in the unit cell dimensions along the [001] direction in rhombohedral PZT and along [101] in tetragonal PZT, confirming the predicted behavior. The findings support a model based on local monoclinic shifts superimposed on rhombohedral and tetragonal displacements, and recent first-principles calculations have successfully reproduced the monoclinic phase and explained the high piezoelectric coefficients.