Perovskite potassium sodium niobates, K 1−x Na x NbO 3 , are promising lead-free piezoelectrics. Their dielectric and piezoelectric characteristics peak near x = 0.5, but the reasons for such property enhancement remain unclear. We addressed this uncertainty by analyzing changes in the local and average structures across the x = 0.5 composition, which have been determined using simultaneous Reverse Monte Carlo fitting of neutron and X-ray total-scattering data, potassium EXAFS, and diffusescattering patterns in electron diffraction. Within the A-sites, Na cations are found to be strongly off-centered along the polar axis as a result of oversized cube-octahedral cages determined by the larger K ions. These Na displacements promote off-centering of the neighboring Nb ions, so that the Curie temperature and spontaneous polarization remain largely unchanged with increasing x, despite the shrinking octahedral volumes. The enhancement of the properties near x = 0.5 is attributed to an abrupt increase in the magnitude and probability of the short-range ordered octahedral rotations, which resembles the pre-transition behavior. These rotations reduce the bond tension around Na and effectively soften the short Na-O bond along the polar axis - an effect that is proposed to facilitate reorientation of the polarization as external electric field is applied.Perovskite-like potassium sodium niobates, K 1−x Na x NbO 3 (KNN), are on the short list of commercially viable lead-free piezoelectrics 1,2 . The technologically-relevant K-rich part of the KNN phase diagram 3,4 is dominated by polymorphic phase transitions that originate in KNbO 3 . This end compound on cooling undergoes a sequence of changes from the high-temperature paraelectric cubic (C) phase to the lower-temperature ferroelectric tetragonal (T), orthorhombic (O), and rhombohedral (R) polymorphs; at room temperature, the O phase is stable. In the average structures, these transitions are manifested by cooperative polar displacements of the cations along the 〈001〉 (T phase), 〈110〉 (O), and 〈111〉 (R) directions of the cubic phase.The temperatures of the C↔T and T↔O transitions remain flat across nearly the entire compositional range. Substitution of K (ionic radius 1.64 Å 5 ) by smaller Na (1.39 Å) eventually induces additional phase changes associated with octahedral rotations, which occur for x ≥ 0.6. The K-and Na-rich parts of the diagram are thought to be divided by a morphotropic phase boundary (MPB) at x ≈ 0.5, which separates the ferroelectric K-rich O and Na-rich monoclinic M phase fields. The piezoelectric and dielectric properties peak at this boundary 6-8 and, therefore, most studies dealing with the development of practical KNN ceramics have focused on the x ≈ 0.5 composition. Even more significant enhancement of properties can be achieved by doping KNN with other species to shift the T-O transition down to room temperature 2 . According to the published phase diagram 2-4 , the untilted M structure is stable over a narrow compositional range between x ≈ 0.5...