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 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....)
The answer to your question is **yes**.

The notation you use is relative to tonic. Establishing which sound is the tonic (and whether it changes) is completely outside of this tool. Consider anything noted as II-V as carrying a standard disclaimer "assuming that X is the tonic".

If you have a song with chords: C F C F C F, it might be a I-IV-I-IV, it might be a V-I-V-I. Depends on the rhythm, melody and even the listener. You can "make" yourself hear different tonics for the same progression in a similar way you can make yourself hear different "1" in a repeated drum loop.

There's some modern research concerning this, the classical theory just assumed that all people have roughly the same musical background and follow the same convention.