A sound box is found in most string instruments.[2] The most notable exceptions are some electrically amplified instruments like the solid body electric guitar or the electric violin, and the piano which uses only a sound board instead. Drumhead lutes such as the banjo or erhu have at least one open end of the sound box covered with animal skin (or a skin-like acrylic material). Open back banjos are normally used for clawhammer and frailing, while those used for bluegrass have the back covered with a resonator.