# Interval analysis, augmented 5th or a 6th?

I am doing interval analysis as I am writing a countersubject(working on the harmony for it right now). But I have a bit of trouble here. I have 3 note choices for any given quarter note. The spot I am currently analyzing has A major harmony. Now because it has a D, A is the only option that isn't dissonant in the sense that it isn't a chord tone. But that D goes to a C# And C# to A is an augmented 5th.

How do I know? C# aug has these 3 notes in order: C#, F, A. But at the same time C# is the 6th degree of the A major scale so it is a 6th as well.

Would I note that interval from C# to A as a 6th or an augmented 5th in my interval analysis? I mean, in A major, it is a 6th. But I have always noticed that the minor 6th alone sounds dissonant. So it sounds like an augmented 5th. If I play just C and Ab alone in a 6th interval, I hear C aug, even if the 6th is technically consonant. Only way I can tell right away if it is augmented or not is if the full triad is there. Major 3rd and it is augmented. Minor third and it is 1st inversion major.

So, if I hear a minor 6th by itself as an augmented 5th, should I treat it as such in 2 voice counterpoint and have the ambiguous resolution of an augmented chord? Or should I treat it as a 6th in a major chord and treat it just as I would a major 6th, even if I hear an augmented 5th? In oher words, should I only treat it as an augmented 5th if I am:

1) Modulating

and

2) Have 3 or more voices in the modulation

• "C# aug has these 3 notes in order: C#, F, A" -- not quite: a C# aug triad is C# E# G##.
– user39614
Nov 1, 2018 at 4:19
• Yes but G## is played the same way as A on a piano and has the same relationships. Bb would still be a minor second from G##. Since I am composing for the piano, I typically think of which enharmonic equivalent has fewer accidentals. Thus I think of the G## in C# aug as an A instead of G##, even though G# is the 5th in C# major. And in the case of C aug, I think things out easier in flats for the black keys in C minor and thus I think of the 5th in C aug as Ab. And even if technically the 5th in Bb aug is Gb, I think of that note easier as F#. Nov 1, 2018 at 4:26
• @Caters That can become extremely confusing. Nov 1, 2018 at 4:45
• I personally find lone, exposed minor 6ths to sound quite consonant. Parallel minor 6ths sound even better (none of that strange sound I hear from parallel 4ths or 5ths). Nov 1, 2018 at 5:26
• When written, C#>A isn't an aug5th. Inverting makes it a well-known maj3, so with the rule of 9, C#>A is a m6th. If you wanted it to be +5, it would be written C#>Gx. An aug. interval often means the following harmony could be, in this case, F# maj. Context!
– Tim
Nov 1, 2018 at 9:45