Sympathetic string

Sympathetic strings or resonant strings are thin auxiliary strings found on many Indian musical instruments, as well as some Western Renaissance-era instruments. They are typically not played directly by the performer (except occasionally as an effect) but instead resonate along with the main strings played on an instrument. The effect 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.