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For naming intervals I have two strategies in my head.

STRATEGY #1

  1. First identify the generic interval between the two given notes by counting the notes between them (inclusive of those two given notes themselves)
  2. Look at the root note. Imagine the major scale for that particular note. If the given second note belongs to the major scale, then the interval is major and I also write the identified interval number detected in step 1. But if the note does not belong in the imagined major scale then it depends on the number of the interval. e.g. if it's fifth, then it can be only diminished, augmented, or perfect.

An example would be C Gb

  • Generic interval is 5th.
  • Therefore the interval quality can be either dim, aug, or perf.
  • C major scale is C D E F G A B C
  • Gb does not belong there, G belongs. Gb is half a step lower, therefore, it's diminished
  • The interval is dim 5th.

STRATEGY #2

Using the charts provided by www.musictheory.net. i.e. counting the number of half-steps included between the two given notes and see where they fit in the charts below.

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Now I have got two questions:

  1. Are both strategies equally good? Are they prone to errors? Can any of them fail for a particular interval?
  2. Are there other faster/better strategies?
  • If you know the letter-names of each note, interval-naming is trivial. I think the problem arises when the notes are merely picked out on a keyboard - IS that black note F# or Gb? - or worked out by fret-counting on a guitar - it's not uncommon for a guitarist to quote a simple chord sequence as e.g. F, C7, A#7... – Laurence Payne Jan 15 '17 at 15:21
  • There does not exist an interval like a first or an eighth. – Neil Meyer Jan 15 '17 at 17:05
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    @Neil Meyer: In many books they do, and I know they are also referred to as prime, unison, perfect in other books. It highly depends on the system you're working with (French, Germany, English, etc.) – xusi Jan 15 '17 at 17:20
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    There's unison and octave though. Perfect, diminished or augmented. C to C# is an augmented unison. – Laurence Payne Jan 15 '17 at 17:40
  • @LaurencePayne For the specific example, what if I wanted to write a Cmaj7♯11 chord? I wouldn't use G♭, because I like having friends, to paraphrase Julian Cianciolo. I must argue that interval naming is not exactly trivial. Suppose I were to write a song, and gave you the chords, but all of them were spelled atrociously, say B♯-F♭-A♭♭ for a C major triad? Or say I tried to explain some concept to you, but said that the fifth of B major is G♭? Yes, guitarists will do that sort of thing, but it really does hinder understanding when communicating between musicians. – user45266 Sep 19 '18 at 0:28
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The first strategy is a lot better than the second: some intervals have different names, but the same amount of half-steps. For example: C-F# would be called an augmented fourth, C-Gb would be called a diminished fifth, but look at the chart! Both are 6 half-steps. This is because step one from Strategy #1 is missing.

I think, as long as you need a strategy (probably you will internalise the identification of intervals fairly quickly), Strategy #1 is a wonderful method, especially because of this first step; it makes sure you understand the difference between enharmonic equivalents (two notes that would be the same key on a piano, but with different names - F# and Gb for example). This difference is the underlying issue in many of the questions here on Music.SE.

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