Tuning fork
The fork shape produces a very pure tone. Most of the vibrational energy is at the fundamental frequency, with very few overtones (harmonics). This is not the case with other resonators. The reason for this is that the frequency of the first overtone is about 52/22 = 25/4 = 6¼ times the fundamental (about 2½ octaves above it). By comparison, the first overtone of a vibrating string is only one octave above the fundamental. So when the fork is struck, little of the energy goes into the overtone modes; they also die out correspondingly faster, leaving the fundamental. It is easier to tune other instruments with this pure tone, when listening to compare with the tone of each other instrument.