Just adding notation visuals - otherwise @Richard answered the question.
You must consider both key signatures and accidentals.
With no key signature and no accidentals, B natural and D natural, minor third
With no key signature and accidental sharp on D, B natural and D sharp, major third
With key signature of 5 sharps and no accidentals, B natural and D sharp (from the key signature), major third
With key signature of 5 sharps and accidental double sharp on D, B natural and D double sharp, augmented third (enharmonically a perfect fourth)
Key signature of two flats and accidental sharp on D, B flat (from the key signature) and D sharp, augmented third (enharmonically a perfect fourth)
Key signature of two flats and no accidentals, B flat (from the key signature) and E flat (from the key signature), perfect fourth
- Minor third = 3 half steps
- Major third = 4 half steps
- Augmented third/perfect fourth = 5 half steps
Notice how the combination of key signatures and accidentals of notation make clear and unambiguous whether two pitches are one of two enharmonic possibilities.
A small addition to address directly the OP's original scenario: a
B majro key signature with notes
B natural and
D sharp notated.
one note on B and the other was on D sharp, would that mean it would be an augmented third, because of the key [B major]?
This is literally what the question posed. The sharp on the
D is redundant, because there is already a sharp in the key signature. It's the exact same meaning as...
D sharp had been altered, then the sharp could be used to restore it back to the
D sharp of the key signature.
D was lowered with a natural sign, it will stay
D natural until the end of the bar. If you want to set it back to
D sharp before the end of the bar, add the sharp.
When the bar with
D natural ends, the
D is assumed to go back to the key signature as a
D sharp. A sharp in parenthesis can be used after a new barline and reminder. This is called a courtesy accidental.