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I guess it's an augemented fourth because it contains a half step more than a perfect fourth; [from a source] but in a minor scale it isn't allowed: an augmented fourth is IV degree.

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    Interval naming has nothing to do with scales. It can be convenient to think of interval names in terms of the Major scale, but interval names are only about the distance between note names.
    – user39614
    Mar 1, 2018 at 2:10
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  • Another possible duplicate
    – user39614
    Mar 1, 2018 at 2:36
  • @David Bowling - Would 'interval numbers are about the distance between note names' be more accurate?
    – Tim
    Mar 1, 2018 at 9:02
  • @Tim -- probably. Maybe 'interval names are are about distance and can be found by using the distance between note names and the distance between notes in semitones together in combination.' It seems difficult to capture an accurate description in a single sentence.
    – user39614
    Mar 1, 2018 at 13:49

3 Answers 3

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Any A to any D is some kind of 4th since the distance in letter names is that of a 4th. Since A to D is a perfect 4th, making the interval bigger by one semitone from A to D♯ makes this interval an augmented 4th.

For more information about naming intervals in general, please see one of the many questions on the site addressing this:

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Depends on which note is the lower. From D# to A, one has D#,E,F,G,A (with 5 letters) the interval is a diminished fifth. Going from A to D# one has A,B,C,D# and 4 letters so it's an augmented fourth.

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I've always referred to that interval as the Tritone and folks seem to know what I'm talking about. Augmented fourth or diminished fifth offer an explanation of how we arrive at this result, but I know it as a Tritone interval, am I wrong?

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    Written as A > D#, it's an aug.4th. Written as A > Eb, it's a dim5th. Either way, it's going to be a tritone, but quoted here, it's more specifically the former.
    – Tim
    Mar 1, 2018 at 14:55
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    Also note that an augmented 4th and a diminished 5th are the same pitch in equal temperament, but not necessarily in other tunings, so tritone is less precise.
    – user39614
    Mar 1, 2018 at 15:05
  • Although it's often convenient short-hard in casual settings when everyone knows more or less the context, technically speaking tritone is not an interval name - just a measure of the distance between two notes. An interval contains musical information. Augmented: a major or perfect interval increased by one additional half-step; 4th: The 4th note in the scale starting from the first note.
    – Stinkfoot
    Mar 2, 2018 at 20:48
  • @Stinkfoot- I'll keep that in mind next time, it's good to know. Mar 4, 2018 at 5:29

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