A plastic violin is provided with means for compensating for change of tension in the violin strings due to dimensional instability of the plastic caused by variations in temperature or pressure. The plastic violin comprises an elongated bar below the finger board and is bonded thereto. An elongated stabilizing bar is placed in the hollow portion of the violin body and has a proximal end which abuts an upright portion or identation in the reinforcing bar and a distal end which is received by a threaded compensating screw which is maniputable to bias the proximal end of the stabilizing bar against the surface of the indented upright portion of the reinforcing bar.

https://patents.google.com/patent/US4809579A/en?inventor=mario+maccaferri&oq=mario+maccaferri&page=2