Some additional examples:
If we treat the bottom note of any interval as the tonic of a major scale, we can compare the top note to members of the scale.
For each example, click on the interval name in the text to hear the interval.
In the first measure- F would be the tonic (F major) and Bb would be scale step four. Since it is a B flat the interval is a perfect fourth (P4).
In the second measure-E would be tonic (E major-four sharps). The upper note is scale step six. The C would be sharp in E major, so we know the interval is a major sixth (M6)
In the third measure- A flat would be the tonic (A flat major-four flats). Since the top note is scale step six, and F would be natural in A flat major, we know that the interval is a M6.
In the fourth measure- D flat is tonic (D flat major-five flats). The upper note is scale step five which should be flat in D flat major. Since the A is natural, the interval in one half step larger than perfect, and would be an augmented fifth (A5).
NOTE: If the bottom note can't be the tonic of a major scale (ie: it isn't on the circle of fifths), you must change it to a note that can be tonic, and change the upper note the same amount.
Example: The F flat is not a tonic of a major scale- there is no F flat major.
So we lower both notes by a half step to make the F natural and the B flat becomes B double flat. Now we can use F major (scale step one up to scale step four). Since B double flat is a half step lower than B flat, the interval is a half step smaller than perfect - so it is a diminished fourth (d4).