The shear piezoelectric behavior in relaxor-PbTiO3 (PT) single crystals is investigated with regard to the crystal phase. High levels of shear piezoelectric activity, d(15) or d(24) >2000 pC N-1, are observed for single-domain rhombohedral (R), orthorhombic (O), and tetragonal (T) relaxor-PT crystals. The high piezoelectric response is attributed to a flattening of the Gibbs free energy at compositions close to the morphotropic phase boundaries, where polarization rotation is easily accomplished by applying a perpendicular electric field. The shear piezoelectric behavior of perovskite ferroelectric crystals is discussed with respect to ferroelectric-ferroelectric phase transitions and the dc bias field using a phenomenological approach. The relationship between the single-domain shear piezoelectric response and piezoelectric activities in domain-engineered configurations is also given in this paper. From an application viewpoint, the temperature and ac-field drive stability for shear piezoelectric responses are investigated. A temperature-independent shear piezoelectric response (d(24), in the range of -50 degrees C to the O-T phase-transition temperature) is thermodynamically expected and experimentally confirmed in orthorhombic relaxor-PT crystals; a relatively high ac-field drive stability (5 kV cm(-1)) is obtained in manganese-modified relaxor-PT crystals. For all thickness shear vibration modes, the mechanical quality factor Qs are less than 50, corresponding to the facilitated polarization rotation.