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The consonance of the 4th interval depends on the context, as several music theory sources describe. Why isn't that idea generalized to all intervals? Let me illustrate this comparing what I have in mind for the 4th and the 5th.

If the interval G-C is played, the G can be heard as the tonic, which makes the ear expect a pull C-B to resolve that suspended interval (it's like there's a hidden Gsus4). If the C is heard as the tonic, everything seems stable (most references categorize the 4th as a dissonance even if the C is heard as tonic, when the 4th interval is made with the bass note, which means a second inversion triad. Maybe because I wasn't exposed enough to common practice music, my ear don't feel the extreme urge to resolve the so called instability of a second inversion triad)

If the interval F-C is played, when F is heard as the tonic, we have a stable interval. But wouldn't the reasoning be exactly the same if C is heard as the tonic? In this case there's an expected plagal pull F-E and shouldn't the books list the consonance of perfect 5th as an "interval that depends on the context"? Why people put just the 4th in this special place?

Vincent Persichetti (Twentieth Century Harmony) also categorizes the augmented 4th as an interval of this type and goes even further stating that the consonance depends on the surroundings. Some passages are exhibited here:

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Don't all intervals have their degree of consonance dependent on the surroudings and context? A major 2nd sounds more consonant if all that came before was a sequence of minor 2nds, for example.

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  • This appears to be a great question and I am not equipped to give a good, complete technical answer to it. But I think the key lies with this: my ear don't feel the extreme urge to resolve the so called instability of a second inversion triad - our musical sensibilities and tastes have changed over time - jazz and rock, which break all the old rules, have become ubiquitous over the last 50 or 60 years and so our ears hear music differently than people who grew up 80 or 100 years ago - we expect what was once called dissonance and we don't hear it as dissonance.
    – Stinkfoot
    Commented Feb 19, 2018 at 1:32

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Historically, the fourth is a dissonance when sounded against the bass but not against upper notes. Thus, taking the 1-3-5 chord as consonant (C-E-G), its associates (not associated until later after Rameau), E-G-C is consonant with its third and sixth (the G-C fourth occurs in the upper voices) but the G-C-E chord is dissonant with the fourth against the bass. During the figured bass period, the chord C-E-A was often considered more closely related to C-E-G chord than E-G-C because of the shared bass. Still, moving from C-E-A to C-E-G or vice versa is a common compositional practice. Dissonant doesn't mean "ill sounding" rather it means "signals movement" which is at least as much cultural as sonic.

I have read somewhere that the fourth was considered dissonant as that interval (ratio of 4/3 over the bass), does not occur anywhere in the overtone series. I haven't checked to see if this is true nor if that should have any bearing on the case.

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  • Dissonant doesn't mean "ill sounding" rather it means "signals movement" Thanks for that - important. But: which is at least as much cultural as sonic - are you saying that "signals movement" is "cultural"?! "Ill sounding is "cultural". "signals movement" is objective. Perhaps I misunderstood your intent?
    – Stinkfoot
    Commented Feb 19, 2018 at 3:08
  • It may be universal that in any esthetic description, there will be a way to signal movement and one to signal repose. However, which way isn't guaranteed. Of course, the idea that "octaves can rest" and "minor sevenths must move" isn't absolute and the boundary between rest and movement may be fuzzy. (Often the case in artistic endeavors.) In jazz or popular or RIchard Strauss, the use of lots of seventh chords doesn't imply movement rather just color and texture. I Byrd or Palestrina, different distinctions are made.
    – ttw
    Commented Feb 19, 2018 at 4:13
  • I didn't understand word of your comment - how does that answer my question? If understand correctly, you said that signals movement is 'cultural'. It is not - it is a physical phenomenon. ill sounding is cultural - what sounds 'good' to one culture might sound "bad" to another.
    – Stinkfoot
    Commented Feb 19, 2018 at 4:45
  • That's fine, but it's not related to my specific doubt: why the reasoning I mention is not consistently applied to other intervals?
    – Allan Felipe
    Commented Feb 25, 2018 at 2:48
  • Perhaps the consensus of theorists (The Taxonomists of Music) seems to be that all intervals are dissonant or consonant depending on context. It's just that the fourth seems to have the biggest change for a small context change (C-E-A vs C-F-A).
    – ttw
    Commented Jun 17, 2018 at 15:16

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