Sympathetic string

The resonance is most often heard when the fundamental frequency of the string is in unison or an octave lower or higher than the catalyst note, although it can occur for other intervals, such as a fifth, with less effect.

In Western music, some members of the viola family appeared in the middle of the 17th century that were fitted with an extra choir of thin wire strings running through a hollow chamber through the neck of the instrument, the head of which was then elongated to accommodate as many extra tuning pegs as necessary.

These were generally called viola d'amore; another historical example is the baryton, for which Haydn wrote many trios.

In keyboard instruments like the piano, the string dampers can be raised to produce this effect.

Two tones of the same pitch will give maximum sympathetic resonance as all harmonics of both strings will overlap.