Here's how you can determine two notes' interval:
Remove the notes' accidentals and count the C-major scale's steps between them (e.g. E to B♭ becomes E to B and is five notes).
Identify the interval (e.g. a perfect fifth).
Adjust for the accidentals (e.g. the B♭ has a flat, so the interval becomes a diminished fifth).
Voila! You have the interval between two notes!
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