High-resolution x-ray powder diffraction measurements on poled PbZr1−xTi xO3 (PZT) samples near the morphotropic phase boundary (MPB) reveal that the piezoelectric elongation of the unit cell does not occur along the polar directions but along those associated with monoclinic distortion. This study provides the first direct evidence for the origin of the high piezoelectric response in PZT. The PZT system, known for its excellent electromechanical properties near the MPB, has been studied extensively. Theoretical models have failed to explain the high piezoelectric response in PZT, suggesting the presence of a crucial missing ingredient. The discovery of a stable monoclinic phase near the MPB offers a new perspective on the rhombohedral-to-tetragonal phase transformation in PZT and similar systems. This monoclinic phase plays a key role in explaining the high piezoelectric response in PZT. Experimental evidence from high-resolution x-ray diffraction measurements shows that the unit cell elongation occurs along directions associated with monoclinic distortion, not the polar directions. The results confirm that the piezoelectric strain in PZT near the MPB is not along the polar directions but along those associated with monoclinic distortion. This work supports a model based on the existence of local monoclinic shifts superimposed on rhombohedral and tetragonal displacements in PZT. Recent first-principles calculations have been able to reproduce the monoclinic phase and explain the high piezoelectric coefficients by considering rotations in the monoclinic plane. The high-resolution powder data provide key information to understand the piezoelectric effect in PZT, allowing accurate determination of unit cell elongation along the electric field direction. The study also highlights the importance of monoclinic distortion in the piezoelectric response of PZT and similar materials.High-resolution x-ray powder diffraction measurements on poled PbZr1−xTi xO3 (PZT) samples near the morphotropic phase boundary (MPB) reveal that the piezoelectric elongation of the unit cell does not occur along the polar directions but along those associated with monoclinic distortion. This study provides the first direct evidence for the origin of the high piezoelectric response in PZT. The PZT system, known for its excellent electromechanical properties near the MPB, has been studied extensively. Theoretical models have failed to explain the high piezoelectric response in PZT, suggesting the presence of a crucial missing ingredient. The discovery of a stable monoclinic phase near the MPB offers a new perspective on the rhombohedral-to-tetragonal phase transformation in PZT and similar systems. This monoclinic phase plays a key role in explaining the high piezoelectric response in PZT. Experimental evidence from high-resolution x-ray diffraction measurements shows that the unit cell elongation occurs along directions associated with monoclinic distortion, not the polar directions. The results confirm that the piezoelectric strain in PZT near the MPB is not along the polar directions but along those associated with monoclinic distortion. This work supports a model based on the existence of local monoclinic shifts superimposed on rhombohedral and tetragonal displacements in PZT. Recent first-principles calculations have been able to reproduce the monoclinic phase and explain the high piezoelectric coefficients by considering rotations in the monoclinic plane. The high-resolution powder data provide key information to understand the piezoelectric effect in PZT, allowing accurate determination of unit cell elongation along the electric field direction. The study also highlights the importance of monoclinic distortion in the piezoelectric response of PZT and similar materials.