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I'm trying to learn music theory while learn some simple rock songs, such as AC/DC's. For example "Highway to Hell" and "Girls Got Rhythm".

I know these notes are used: G - A - B - C - D - E - F# - G, which looks like a G major scale and I also read that AC/DC is known for using the mixolydian mode. For this G major, this would mean a D mixolydian mode.

But why is it D mixolydian and not E aeolian? Or any other relative mode? I try to find a concrete answere, not something that relies on "feeling". Like what is the moment in those songs that proves it is D mixolydian?

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    'I try to find a concrete answers, not something that relies on "feeling"' - This is understandable and also problematic when analyzing any kind of art. There is no part of music theory that does not eventually get down to how something sounds and how it feels. This means that music theory is always at least a little bit subjective. Commented Apr 8, 2023 at 19:20
  • Ultimately the feeling is what matters. A concrete, logical answer would just be trying to categorize what causes the feeling. The feeling largely depends on what music you've been exposed to prior to listening to the song. I (rock, metal, prog, marching percussion background) have definitely had disagreements over e.g. what the time signature of a song is with someone who had a different background (e.g. classical).
    – Edward
    Commented Apr 8, 2023 at 19:24

3 Answers 3

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Finding the "tonic"

"The" moment is most often going to be the final chord. More often than not, a song ends at a moment of rest, where the music feels like it has arrived "home". The root of that chord will be the home bass for the mode.

"Highway to Hell", for example, ends on an A. (Hear, for example:

)

Given the notes listed in the question, that would suggest A Dorian. (It's not — the song uses C# more than C, making it A Mixolydian — but Dorian would be the conclusion based on the information given in the question.)

Characteristic sound

Each diatonic mode has its notes and intervals that distinguish it from the others. So to get the sound of that mode, the music will tend to emphasize its unique features.

We know "Highway to Hell" is not Lydian, for example, because it doesn't call attention to the #4 Lydian has relative to major. It's not Phrygian, because it doesn't make use of the b2.

The problem of modes and popular music

"Highway to Hell" is in A Major; however, it avoids G# in favor of G. This is a characteristic quirk of rock. When the song goes to the V chord, it uses the leading tone rather than the lowered seventh of mixolydian. That consistent presence of the leading tone is what makes the song major despite the frequent use of the b7.

One common place for the b7 to appear is in the VII chord itself. This is often used to move to the I chord (the "backdoor cadence"). A song that uses this progression and avoids the V chord with the leading tone (i.e. avoids the chord altogether or uses the minor V) could be more properly said to be in Mixolydian.

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  • "the song uses C# more than C" - I'm trying to find the C# in this song, but I can't. But C is definitely used during the solo, which I understood uses notes from both A Major Pentatonic and A Minor Pentatonic. Commented Apr 9, 2023 at 12:32
  • @AcidMammoth It's in the A chords, which are clearly major rather than minor.
    – Aaron
    Commented Apr 9, 2023 at 15:28
  • Yeah... I heard them as power chords (just A and E notes), but I guess a C# can be heard sometimes (or is just that the chord overall "sounds" major in the context of the song?). Commented Apr 9, 2023 at 17:58
  • @AcidMammoth I'm inclined toward the latter. The C# might just be an overtone or something, but clearly singing a C# against the chord "works" where C doesn't.
    – Aaron
    Commented Apr 9, 2023 at 18:09
  • Another way to view the problem of rock in terms of major versus minor is often there are elements of both, similar to blues and jazz and some Beethoven. Commented Apr 9, 2023 at 20:11
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The first thing is checking wich note would turn as tonic, wich notes avoid as the possible 2nd or 7th, i knwo it is very basic, so as alternate answer, We can see it from the guitar perspective, maybe play it on the guitar and check if it fits into the digitation of certain mode.

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The 'home' chord is the most telling part of what a key is. In Highway to Hell, it's definitely an A chord - it re-appears all through the song, and is used as the last chord. The next tell is that E chord - the dominant, which most often heralds the root/home chord by a feeling of building up to reach it - and that's present too.

So far, it's in key A. But there's that G chord ever present, which isn't diatonic to key A. The G note itself tells us that. Even though it's there in the E chord (so much distortion it's hard to tell!).

So, the song is more likely in A Mixolydian, although the key signature when written could be either 2 or 3 sharps - that's down to whoever writes it. With, of course, the appropriate accidentals either ♯ or ♮ on the G notes involved. Suffice to say, with modern pop music 'it's in A'.

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