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 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.