Ferroic crystals contain domain states which can be switched by suitably chosen external forces. The domains in a primary ferroic differ in spontaneous polarization (ferroelectricity) spontaneous magnetization (ferromagnetism), or spontaneous strain (ferroelasticity), while those in a secondary ferroic differ in induced strain, induced magnetization, or induced polarization. Examples of ferrobielastics, ferroelastoelectrics and other secondary ferroics are discussed, together with their applications as multi-domain piezoelectric resonators. Twinning is often considered a nuisance, but it can be used to enhance certain modes of motion, or to eliminate others, by making use of differences in the piezoelectric coefficients. Multi-domain crystals and ceramics can also be used to produce “forbidden” harmonics, or as high-frequency resonators, and as focused acoustic transducers.