In classical music from Western culture, an augmented third (Playⓘ) is an interval of five semitones. It may be produced by widening a major third by a chromatic semitone. For instance, the interval from C to E is a major third, four semitones wide, and both the intervals from C♭ to E, and from C to E♯ are augmented thirds, spanning five semitones. Being augmented, it is considered a dissonant interval.
Its inversion is the diminished sixth, and its enharmonic equivalent is the perfect fourth.
The just augmented third, E♯, is 456.99 cents or 125:96. Playⓘ The Pythagorean augmented third, E♯+++, is 521.51 cents or 177147:131072, eleven just perfect fifths. Playⓘ
References
Benward & Saker (2003). Music: In Theory and Practice, Vol. I, p.54. ISBN 978-0-07-294262-0. /wiki/ISBN_(identifier) ↩
Hoffmann, F.A. (1881). Music: Its Theory & Practice, p.89-90. Thurgate & Sons. Digitized Aug 16, 2007. Archaically: superfluous third. ↩
Benward & Saker (2003), p.92. ↩