Temperature-driven structural transformations in Pb-based perovskite-type relaxors are studied by using polarized Raman spectroscopy, high-resolution powder, and synchrotron single-crystal x-ray diffraction applied to PbSc 0.5 Ta 0.5 O 3 ͑PST͒ and Pb 0.78 Ba 0.22 Sc 0.5 Ta 0.5 O 3 ͑PBST͒. The two compounds were chosen as model systems because PST is a relaxor that exhibits ferroelectric long-range order on cooling, whereas PBST shows canonical relaxor behavior. The temperature evolution of phonon anomalies and the pseudocubic unit-cell parameter for both PST and PBST reveals the existence of a characteristic temperature T ‫ء‬ between the Burns temperature T B and the temperature of the dielectric-permittivity maximum T m. T ‫ء‬ is associated with the coupling of initially nucleated small polar clusters and their aggregation into larger polar clusters. The temperature range between T B and T ‫ء‬ is characterized by a coupling between adjacent off-centered BO 6 octahedra to form initial polar clusters, while the range between T ‫ء‬ and T m is characterized by a coupling between off-centered B cations from adjacent polar clusters. Off-centered Pb atoms exist even above the Burns temperature and their length of coherence governs the coupling between polar regions comprising B-cation offcenter shifts and, consequently, directs the formation of the ferroelectric state to a normal, long-range ordered or nonergodic relaxor state.