I’ll be Your Mirror
by The Velvet Underground
is most likely in the Key of A#/Bb Major
Notes in A#/Bb Major:
A, A#/Bb, C, D, D#/Eb, F, G
Chords in A#/Bb Major:
Bb, Cm, Dm, Eb, F, Gm, Adim
This key can also be known as G minor.
When it's used as a minor key, the minor key note is stressed more than the major key note. The same notes and same chords still apply.