A stringed musical instrument, such as a guitar, mandolin, or a violin, has an elongated frame to which modular components such as necks, electronic pick-ups, electronic control boxes, bridges, tail pieces, decorative components, or body parts can be adjustably placed and clamped. This not only allows the position of many such components to be customized, but also allows different components to be easily substituted for each others; allows the instrument to be quickly taken apart for more compact storage or shipment; and allows the instrument to be changed so as to produce a different sound or look. Preferably some the components, including the body parts, are rotatably mounted relative to the frame, so their angle relative to the plane of the fingerboard can be adjusted to suit a user's tastes or physical disabilities, such as carpel-tunnel stress syndrome. In some embodiments, an entire string assembly, comprised of a set of strings connected between a head and a bridge or tail piece, can be modularly attached to or replaced from the instrument.