Resonant strings are thin auxiliary strings found on many (Asian) 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 musician retunes the resonant (often referred to as "sympathetic") strings for each mode or raga, so that when the corresponding note (or one an octave below it) is played on the main strings of the instrument, the resonant strings vibrate sympathetically, providing a sound analogous to our modern echo chamber, and aiding in the instrument's projection.
The same effect can be performed on a modern piano, by softly striking a key in the lower register of the piano and immediately striking that note in a higher octave.
