I’m going to disagree with everyone here. As far as I am concerned, the intro (everything before the Em on “just”, at which point we’ve changed to the key of D) is actually in the key of F# (or F# Lydian, depending on whether you interpret the C natural in the melody, on the first syllable of “understand ”, as diatonic or chromatic). When I say “everything”, there is of course the qualification that the D chord is a chromatic chord (a fleeting excursion outside the key). But that is equally true if you analyse it in either of the other two ways seen above.
And yes, I know what you are thinking. An F# chord is never played at any point in this song, how can that be the key of the intro. The genius of this song is that the key change happens BEFORE the intro has had a chance to resolve. It never gets to its tonic chord.
Determining the key is a job for the ears, not the analytical part of the brain that reads key signatures and counts how many sharps and flats there are. The tonic chord is the chord that is implied as the place of rest, that feels like “home” if and when it is played, and none of the four chords that are actually played in this sequence cut it. Stop on any one of those chords and it doesn’t feel resolved. We still have somewhere to go. But bail out of the sequence at almost any point and play an F# major chord, and you’ll realise that was the implied place you were heading all along. You just never get there because the key change to the key of D happens first.
As I said… If you want to be pedantic, let’s call it F# Lydian, if it’s important to anybody to keep that C natural in the melody as part of the key signature rather than an accidental. Anyway, don’t READ the key. HEAR the key. Use your ears! They’re much more trustworthy. Seriously, does anybody feel like they’ve reached “home” on the third chord of that sequence (Db)? I seriously doubt it.
The Ebm (relative minor of F#) feels a little more resolved than Db, so analysing it that way would make a bit more sense (you could call it Eb Dorian if you want to account for the C natural in the melody). But it still doesn’t feel nearly as resolved, IMO, as it does if you subsequently move to F#.
To my ears, that third chord in the sequence (Db) is clearly the dominant, not the tonic, because the intro is actually in F# or F# Lydian (depending on whether you want to consider that one C natural in the melody as diatonic or chromatic), even though we never actually reach the tonic chord, and the sequence is vi / #V / V / iii. If the key change didn’t happen, there is an expectation that we will eventually resolve to F#, but the key change to D happens before we can get there.
To see what I mean, start playing and singing the song with its existing chords, but then do any or all of the following:
- When you get to “promise to be true”, just extend the “true” note as if it was the end of the song, and rather than moving from the D chord to the Db chord, move instead from the D chord to an F# chord. Then stop. You’re home. As will be obvious to you.
- When you get to “and help me”, extend the “me” note as if it’s the end of the song, and rather than moving from the Db chord to the Bbm chord, move instead from the Db chord to an F# chord. Then stop. You’re home. As will be obvious to you.
- When you get to “understand”, just extend the syllable “stand” into the next bar, moving your voice up a half-step from F to F#, as you move from the Bbm chord to an F# chord (instead of to the Ebm7 chord). Then stop. You’re home. As will be obvious to you.
It doesn’t matter at what point you bail out of the chord sequence to resolve to F#, it’s completely clear that F# is where the sequence WANTS to resolve to, rather than any of the four chords actually IN the sequence. This is even true moving from the chromatic (outside the key) D chord (#V) to resolve to the F# tonic. D is outside the key, but it’s very clear that the key that it’s outside of - the one we instinctively want to get back to - is F#, not Db.
- START with two bars of just the F# chord. Establish it as the tonic right off the bat. Then begin the song. See? The sequence makes sense when you are explicitly thinking of F# as “home”. Any way you slice this, F# is the tonic.
This actually makes the intro even more ingenious and unusual. How many pop songs establish a key without ever actually playing the tonic chord, and then modulate to a new key when they STILL haven’t done so? This is a song with a section in F#, but in which an F# chord is at no point ever played! That seems kind of remarkable in and of itself.
Oh, and one more thing: I would argue the key change to D happens on the Em, not on the D chord immediately before it. Because the D chord occurs exactly where we expect it to because of the repetition, we interpret it as a chromatic chord, which is what it was the first time we heard it. We don’t know we’re about to permanently move to the key of D, so we don’t hear it as a key change per se until the Em happens.