Full discloser: I'm fairly new to music theory.
I'm trying to determine a song's key/scale, and I found this site's key finder tool.
I put in the song's chord progression of b minor, D major, G major, and F# major.
It determined that the song is in b minor. So I wrote out all the notes in the b minor scale and I noticed something.
The song uses a F# major chord, which includes the notes F#, A#, and C#. It sounds good and fits with the song, but A# isn't in the b minor scale.
Looking a bit more into it, I looked at b minor scales' chords and F# major isn't listed, instead, there is a f# minor as its v chord.
Just to be sure I tried playing a f# minor chord alongside the song, but it didn't fit.
Is the song doing something exotic or am I misunderstanding something? I ask this because the F# major chord works/sounds good when my understanding of music theory says it shouldn't work.