Technically, a tuning fork is an acoustic resonator. When struck it produces several tones – a fundamental and at least one harmonic – but the fork’s shape tends to minimize the harmonics and within a few seconds only the fundamental can be heard. The tone a fork makes is determined primarily by the length of its “tines” (or prongs). Longer tines vibrate more slowly and thus produce a lower tone. Shortening the length of the tines allows them to vibrate faster and thus produce a higher sound.