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


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?

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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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.
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.
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!