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: 
The scale/chords are good clues to the key of a song, but at least as important is the *tonal center*. That's not as easy to define but generally it's where the song comes back to a place of less musical tension.

IMHO the chord played when the song in question returns to a "rest" state is A major. Which means I would say the song is in A and the progression is ii - bVII - IV - I. That would suggest that the scale is actually A mixolydian (not pure major or minor), which is not unusual for rock.

The point is that the key of a song is not only about the scale. Learning to hear the tonal center is sometimes more important.
The problem goes away if you consider D and Bm to be two sides of the same key. They have the same key signature. The Roman numeral analysis system - in the form you talk about - assumes that either the major or minor side is clearly more prominent, in order to assign number one to a scale degree. You should make a decision: do you want to subscribe to this idea of two possible "ones", or normalize the situation and have the major side's tonic be number I always, even for tunes that are "in minor". Who cares about the numbers? You know where D and B are, what notes are in the default scale specified by the key signature, and how to play the song, isn't that enough?

The tonal center can sway back and forth in a song, and it's often more or less ambiguous. It can even feel like moving completely outside the original major and minor side tonics, and then we talk about a potential modulation. Insisting on moving the number one around all the time is useless IMO. The whole Roman numeral thing and functional harmony is a simplified model for harmony exercises and theory classes. Just a tool to get a perspective on a static harmonic snapshot. But real music isn't so clear and static, it moves and morphs around.