Like any plucked instrument, mandolin notes decay to silence rather than sound out continuously as with a bowed note on a violin, and mandolin notes decay faster than larger stringed instruments like the guitar. This encourages the use of tremolo (rapid picking of one or more pairs of strings) to create sustained notes or chords. The mandolin's paired strings facilitate this technique: the plectrum (pick) strikes each of a pair of strings alternately, providing a more full and continuous sound than a single string would.
 What parts of the mandolin don't facilitate the tremolo technique?