Strings have harmonic resonances at multiples of the fundamental frequency. The bridge of a stringed instrument transmits these harmonics to a surface that has its own resonance frequencies which may not be harmonic. The surface can move more air than a string and so acts to amplify the sound of the string (at the expense of a shorter vibration time for the string). If the instrument has a hollow body the Helmholtz resonances of the cavity contribute to the frequencies that are present. The particular combination of string, surface and body cavity resonances emphasize a particular collection of overtones for any given note which gives each individual instrument its timbre.