One of the great mysteries of the electric guitar universe is the interchangeable use of the terms “vibrato” and “tremolo.” Misuse is actually more accurate.

The confusion started in the 1950s, when amp and guitar makers began mislabeling their products. Suddenly guitars sprang up with “tremolo” arms and amps had “vibrato” circuitry. That’s backwards. The truth of the matter is that what Bigsbys and other whammy bars in the hands of great guitarists like Johnny A. do is “vibrato” and what amps do when they stutter attractively is “tremolo.”

To clarify: vibrato is always about pitch while tremolo is always about volume.

Vibrato is a variation in the pitch of a note or chord. It can be accomplished with the fingers, a whammy bar or a stomp box or plug-in. Finger vibrato is often a key part of the signature of a great guitarist, like Eric Clapton, for example, who uses the technique brilliantly. Finger vibrato, the practice of lightly moving one’s hand — ideally from the wrist, like B.B. King — or a finger that’s pressed on a note, creates a psychoacoustic perception of warmth and that indefinable but nonetheless recognizable quality called “soul” in one’s playing.

The first vibrato arm for guitar was marketed in the mid-1930s by guitar maker Doc Kaufman and was most notably initially used on lap steels. The first fully functional vibrato arm and tailpiece assembly for the round-neck electric guitar was patented by Paul Bigsby in 1952, although the country music six-string virtuoso Merle Travis was using one of Bigsby’s prototypes in the late 1940s. The Bigsby, as it’s lovingly called, still remains the basic blueprint for the whammy bar we know and love today. The tension of the strings is balanced against a spring that pulls those tuned strings back into proper pitch when the downward pressure on the bar itself is released. Gibson Flying V legend Lonnie Mack used the Bigsby unit to historic effect on his 1963 instrumental smash “Wham!” —creating the nickname “whammy bar” In the process.

Now, on to tremolo. In acoustic music, tremolo is typically accomplished with the finger, by applying and reducing pressure on a string to sound a repetition of a note. For electric guitarists, tremolo is also produced via a circuit in an amp or an effects device. A tremolo circuit automatically turns volume up and down. The degree in those highs and lows is often identified by a “depth” control, while the frequency of the repetition of that effect is the province of the “rate” dial.

Amps with tremolo use several different kinds of technology to achieve the effect, including low frequency oscillators, bias modulation and light dependent resisters. Without getting bogged down in such details, think of the tremolo effect simply as somebody turning a volume pot up and down with superhuman efficiency. The sound is unforgettable once it becomes familiar.

Vibrato and tremolo pedals aren’t as popular as distortion and overdrive devices, but thanks to the explosion of boutique pedals on the market in recent years it’s possible to find the unit you need for anywhere from $25 to $200.