It turns out that a single digital waveguide provides a relatively static-sounding string synthesizer. This is because several coupling mechanisms exist in natural string instruments. The overtones in coupled strings exhibit more interesting amplitude envelopes over time. Coupling between different strings is not the only important coupling phenomenon. In a real string, there are two orthogonal planes of transverse vibration which are intrinsically coupled to each other . There is also intrinsic coupling between transverse vibrational waves and longitudinal waves (see §B.6).