Question: quick question that's been bugging me lately. When it comes to a songs key specifically major and minor relatives- is it correct to say it can be written in two different keys with the same progression with the progression being written differently? I say this because a lot of times pop songs I see people post the progressions for, (usually 4 chord progressions that do not change during the song) some people list the major key and some list the minor key. For example lets use the song "Wake Me Up" -- A lot of people post the songs progression as a VI, IV, I, V and in the key of D. But other websites classify the song as in the key of Bm with the progression I, VI, III, VII.

My personal opinion would be that the song begins with Bm So I would say it's in the key of Bm following a I, VI, III, VII. If it had started on a D and modulated to Bm I'd say its in the key of D--- I'm I correct in saying this? Can you always class the song in two different keys since you can rewrite the progression differently but use the same chords? pls help :(

Answer: 
A complete song need not be just in one key, and stay in that key throughout. I guess that's one good thing about relative key signatures - they are identical.

There are many songs which may be in relative major for the verse, and move to relative minor for the chorus - or vice versa. There are many songs which move between the two relatives *during* the verse/chorus.

Often a telling clue is the dominant harmony - but not always. In the relative minor, often its dominant will use a 'proper' leading note - let's take key C/Am. In key C there's a G note, whereas in key Am, that often gets changed to G♯ - the leading note of *that* key, but not actually part of the diatonic set of notes making up C major. There again, that may just be a move to modulate for a short time *to* the relative minor, or even just to get to a bar or two where Am fits best.

Most pieces have a **tonal centre**, which feels to most like 'home'. This is usually the clue to the piece's key. Does it feel like it can stop at that bar, and be at rest, finished? In a piece such as you proffer, which sounds most like 'at home'? As mentioned in the previous para., a perfect cadence will usually offer the best clue.

Incidentally, *major* chords are written using capital RN, *minors* use lower case.

And with some pieces, it's (almost!) impossible to say what the key is!

Sweet Home Alabama, Unforgettable and Fly me to the Moon come immediately to mind. Maybe someone can convince everyone which key is accurately the 'correct' one for each! But does it *really* matter?

EDIT: to directly answer what I think is the question - let's take a sequence vi, IV, I, V. That could also be written in relative minor as i, VI, III, VII. In any given key, that sequence will be the same chords - e.g. in key C - Am, F, C, G.
The answer lies in which chord is used to finish the piece (or section thereof.) If a piece ends with V-I (with I being D major and V being A major for example), one would say that that section (or piece) is in D major. To end a piece in the relative minor (same key signature but that's just for notational convenience and not universally used during the Baroque), one would have V-i with i being a b minor chord and V being F# major. (The V chords could be V7s without changing the analysis.)

Try playing A7-D or A-D then F#7-b or F#-b and listen to the difference. This applies to Common Practice Period harmony (which is still commonly used) where the chord on the dominant is a major (even if needing an accidental) in a cadence. (A almost said accidentally major....)