The physics of paired strings (AKA Courses)

 Sympathetic resonance is a harmonic phenomenon wherein the string of an instrument responds to external vibrations to which it has a harmonic likeness. Strings in pairs will energize each other due to their proximity, similar to the way a tuning fork rings when struck. A unison or octave will provoke the largest response as there is maximum likeness in vibratory motion. The principle of sympathetic resonance has been applied in musical instruments, such as harps, guitars, pianos, and mandolins.