The theremin is an electronic instrument patented in 1928 by Léon Theremin. The typical theremin users two oscillators - one producing a static frequency, one variable - and amplifies the difference of these two signals to produce sine waves at different frequencies, thus creating different pitches. A theremin uses a secondary circuit to control amplitude. You can control pitch/volume with your hands by adjusting the position of your hands in relation to the two antennas of the theremin, thus making it
one of the only instruments in the world that you play but never touch!
The tone of the theremin is well known, and can be heard in countless horror scores because of its somewhat unnatural and haunting tone. Theoretically, a theremin produces a fundamental pitch as a pure sine wave, and pure sine waves sound unnatural because most sounds in nature come with overtones.
The theremin is considered to be one of the worlds most challenging instruments to play. Not only is a theremin "fretless" like a violin, but even stringless, and the tone it produces is not stopped by inaction or through natural decay, but rather by manually decreasing the volume. In other words, to produces staccato notes, one must control not only the attack and hold, but also the decay. This is counterintuitive to how almost every other instrument works.
In essence, the theremin was the worlds first synthesizer, using electronics to "synthesize" a pure electrical tone. We have endeavored to recreate the sound and feel of this instrument by focusing on things like portamento and vibrato, as without properly simulated technique, a theremin simulator would be little more than a sine-wave generator.