A tremolo effect (not to be confused with vibrato) offers adjustable variations in volume of the original note. Unfortunately, Fender's mislabeling of amplifier tremolo effects as "vibrato" and a guitar's vibrato bar as "tremolo" has only continued the confusion associated with these two effects. Amp-based tremolo was incredibly popular in the 50's and 60's, with amps by Gibson, Danelectro, and Fender incorporating the effect (mislabeled or not) on their gear. The introduction to the Rolling Stone's song "Gimme Shelter" is a perfect example of the effectiveness of the unit, as is the beginning of the Wallflowers' tune "One Headlight." The effect fell out of favor in the 80's and 90's, but is fortunately making a comeback, as a tasteful use of the tremolo effect can add a beautiful texture to a guitar part.