Sympathetic resonance or sympathetic vibration is a harmonic phenomenon wherein a formerly passive string or vibratory body responds to external vibrations to which it has a harmonic likeness. The classic example is demonstrated with two similar tuning-forks of which one is mounted on a wooden box. If the other one is struck and then placed on the box, then muted, the un-struck mounted fork will be heard. In similar fashion, strings will respond to the external vibrations of a tuning-fork when sufficient harmonic relations exist between the respective vibratory modes. A unison or octave will provoke the largest response as there is maximum likeness in vibratory motion. Other links through shared resonances occur at the fifth and, though with much less effect, at the major third. The principle of sympathetic resonance has been applied in musical instruments from many cultures and times. Apart from the basic principle at work on instruments with many undamped strings, such as harps, guitars and pianos with the dampers raised, other instruments are fitted with extra choirs of sympathetic strings, which respond with a silvery halo to the tones played on the main strings.