In the case of a guitar or a banjo, this vibrating body is the string. These harmonics are expressed in terms of ratios of small numbers. Whenever you pluck a string, it will vibrate in all of these modes at once. Normally, the fundamental will be the loudest of the tones, and each of the higher harmonics will be increasingly less strong. It is the amount of each of these harmonics that gives an instrument its unique timbre or tonal quality.
Many choral experts theorize that a capella singers use just intonation.
We tend to hear in terms of just intonation. In instruments that have only 7 notes per octave, i.e. diatonic instruments, such as the harmonica, bagpipe, Celtic harp, lyre, etc. there is no problem tuning the instrument in such a way that all the intervals are just. However, when instruments are chromatic, such as the piano, the guitar or the banjo, things soon go awry. The traditional way to tune a keyboard instrument starts with just intervals. A particular note, such as Middle C is played on a reference instrument, such as a tuning fork.