@David Lieder asked why G# major works in this song when G# minor is what is in scale. the answer i gave didn't fully explain the why. i've read a couple of music theory books since then... so here goes a more in-depth answer...

warning: music theory ahead. those champing at the bit for future music theory installments by @Nate Savage can chew on this... i trust nate and @Jim Mrvos will correct anything i get wrong.

whoa, david... don't dive straight into this... it's gonna be some dry material... hydrate first 🤣

let's start with the chords used in the song: E G#7 F#m A B7 E. if i write that in the numbering system we get I III7 ii IV V7 I. we see at the core the familiar pattern... I IV V7 I. this is a good place to start. why does the I IV V7 I progression work? to explain that we need to talk about chord functions, and which means we need to start with intervals and unstable notes in a scale.

i'll use C major for the discussion b/c it has no sharps or flats. also, we can avoid talking about a B#. the chord progression for the song in C major would be C E7 Dm F G7 C.

for reference, here are the scale degrees / notes / chords for C major:

C Dm Em F G Am B C

I ii iii IV V vi vii I

note that between scale degree iii (E) and IV (F) there is only a half step. the same between vii (B) and I (C). all the other notes have a whole step. thus, the IV and the vii are both unstable and want to "move" to another note. the IV (F) wants to resolve down to a iii (E) and the vii (B) wants to resolve up to a I (C).

I’m going to ignore the IV for a minute and focus on the V. the V chord (G in C) contains the most unstable note, the vii (B in C), as it’s major 3rd. this chord wants to resolve that B to a C, giving the familiar V-I ending.

if you add a dominant 7th to the V chord (the only chord in a scale that has a naturally occurring dominant 7th) you get a chord that contains both unstable notes (e.g., B and F in C) and provides a very strong pull back to the tonic. a dominant 7 chord often signals the key signature to our subconscious. the V chord is known as the dominant, and is core purpose is to create a strong tension that resolves to the tonic.

play a G7 chord and then a C chord and you’ll hear how strong the resolution is. the V7 I progression is so strong that a key can be determined just by those two chords. note that if you add together the notes in the V7 and I chords, you get every note in the scale except the vi, which is the relative minor, and can be inferred from the I.

the dominant 7th is so strong that it can also resolve to a minor chord. thus G7 can resolve to C or Cm. why? the 3rd of the V chord is a half step away from the tonic. it is called a leading tone, and both the V and V7 chords want to resolve it to the tonic, regardless of whether it is I or i (C or Cm). the only difference in C vs. Cm is whether the 3rd is flattened. so while the G7 to Cm resolution is not quite as strong as the G7 to C b/c the F resolves to Eb instead of E, the leading tone (B) for the key (C) is resolved in both cases.

there's a bit more to the tension of a V7 chord related to tritones, and we're ignoring things like strong vs. weak beats, but the above interval discussion is the core of why a V resolves to a I.

dominant 7ths are at the crux of why the III7 works. but first...

let’s briefly mention that the IV chord builds tension from the I due to the IV note in the scale (F in C) being unstable and wanting to resolve down a half step. note that the IV chord lacks the leading tone of the scale (B in C) and also shares a note with the tonic.. the tonic itself. finally, the root resolution is a perfect fifth ascending instead of descending. for all these reasons, the IV’s tension is less than the V.

thus, the I IV V7 I progression builds tension from the tonic to the subdominant (I to the IV), builds a little more tension from the subdominant to the dominant (IV to the V7), and then resolves tension from the dominant to the tonic (V7 to the I). a cornerstone of music is increasing, decreasing, prolonging, and resolving tension.

great. so what about the ii and III7?

let's move on to the ii. the ii (Dm in the key of C) can be substituted for the IV (F in the key of C). the reason is the core tone of the tension (F) that exists in the root of the IV is the defining 3rd of the ii. it is common to substitute a ii7 (Dm7) chord for a IV (F) as the ii7 chord contains the IV within it (Dm = D F A C). you've heard me talk about thinking of 7 chords as regular overlapping chords... a ii7 chord is a ii chord (D F A) plus a IV chord (F A C). in the song, you see that exact progression. so the progression goes from the tonic to a subdominant substitute to the subdominant to the dominant and back to the tonic. we see the tension is building slowly toward the subdominant, drawing out the I IV V I tension by adding the ii before the IV.

since the ii and IV work together, let's rewrite the progression I III (ii IV) V7 I.

that only leaves the III, which should be iii if it were in scale. chatgpt will tell you that major chords can often be substituted for minor chords, but why? and when? why does the III7 work here?

first, let's play a shell game. the root is often extended as a maj7 or maj6 chord. also, vi and vi7 chords can act as a minor substitution for the root. why? just like the ii contained the root of the IV, so too the vi contains the root of the I, and the vi7 chord (e.g., A-C-E-G) contains the entire I chord (e.g., C-E-G). in fact, a Imaj6 chord and a vi7 contain the exact same notes. adding a vi chord after a I moves a song from stable tonic to less stable tonic substitute, and builds a little bit of tension.

let's add a vi to the progression to "ease" out of the I. so now we have I-vi-(ii-IV)-V7-I.

time for secondary dominants...

now, remember how the dominant 7 was so strong it signaled a key? secondary dominants are dominant 7 chords that have a root and target in scale, but may include chromatic notes out of scale. secondary dominant are used to resolve to a chord other than the tonic. they prepare the ear for the chord the same way a V7 prepares the ear for the tonic.

these chords are written as V7/ chords. for example, if i wanted the secondary dominant of the vi in a key, i would calculate the V7/vi. in the key of C, the vi is Am, and the fifth in Am is E. therefore i can use an E7 chord in C major to signal a resolution to Am. both the root of the secondary dominant (E in this case) and the resolving note (the A in this case) are in the same scale as defined by the tonic (C in this example). the V7/vi contains a note, it's 3rd (G#), that is not in the C major scale.

finally, note that the V7/vi chord is the III7 key in the major scale. thus in C major, we can use a E7 instead of Em7 when we are preparing the ear for an Am. so now we know where the III7 comes from... it is the V7 of vi.

as a side note, the V7/I is just the V7. you can keep extending this to secondary dominants of secondary dominants, etc. so you could ease into your V7 chord (e.g., G7) with a V7/V7 chord (e.g., D7, or II7) and you could ease into the V7/V7 with a V7/V7/V7 (A7 to D7 to G7... the . the circle of fifths is very handy for this.

secondary dominants are useful for prolonging and "smoothing out" transitions.

b/c we don’t remain on the secondary dominants for long, their chromatic additions to the scale are heard as passing tones. however, if we return to the secondary dominant repeatedly, the ear will hear a key change instead of a passing tone. secondary dominants are also often used to smoothly change keys.

now back to our progression… I-vi-(ii-IV)-V7-I

to smoothly transition from the I to the vi, we can use secondary dominants. in this case, we want the secondary dominant of the vi chord, notated V7/vi. as we just saw, this is the III7 chord. so our progression is now: I-III7-vi-(ii-IV)-V7-I.

done, right?

but, you say, we didn't resolve to Am. we’re not going from I to vi in the song... so why does it work? well, we did resolve to a vi... in a way. we resolved to a IV chord... in this song it is ii + IV (or D F A C in our key of C example). this is called a deceptive resolution. the ear was expecting the vi (A-C-E) but it got the IV (F-A-C). note the overlap between the expected resolution and the deceptive resolution.

deceptive resolutions are used to give music a feeling of progression while prolonging tonic resolution.

so we can see that the III7 was actually preparing the ear for the ii and IV, so let's remove the vi.

so if we look at the whole song again, I III7 ii IV V7 I, we see we begin with the tonic. from the tonic we progress to the III7 which creates slight tension and signals a key change to the vi. we use a deceptive resolution to resolve to the IV instead of the vi, but we delay getting to the IV by using the ii. we then progress to the extreme tension of the V7 and finally release back to the tonic.

hope this explains your question a bit more satisfactorily.

maybe this will spark some intense theory discussions, give someone an idea of how to modify a basic I IV V I to something more interesting, or at the very least, given some food for thought as to why some of the progressions of our favorite songs work.