#1
So I was just transcribing a song and realized it used only these notes:

D E F G A C

Now, the tonic is for sure D.

But as for the B or Bb...

Argument for minor - it doesn't specify the raised 6 (B) so it's safe to just keep it simple and call it minor.

Argument for dorian - it wouldn't be useful to put a key signature to flat a B that is never played, so it would be a clear key signature. Thus the 2 of C major / Ionian. Thus Dorian.

Or is it something else?

#2
The information you've posted tells us absolutely nothing in regards to the answers you're looking for.

What are the chords?
What does the actual piece SOUND like?
Actually called Mark!

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#4
My two cents...
In my opinion, D minor. Why declare something as dorian when there is nothing to indicate it is?
Generally, the simplest solution is the best.

Also, the root is clearly D and there isn't a massive reason to call it key of C to eliminate the one flat in the key signature.
#6
around 1:20. there is a very noticeable B natural, but the chord also changes to A minor. So you could either say that its dorian, or that it modulated from D minor to A minor for a few bars, because it ends up coming back to D minor.
#7
Haha. I hear ya. Checked my music and sure enough... I had a B natural. Thanks man. I'm face palming right now.

Thanks a lot!