Perfect intervals are different from major intervals in that they have only three possible qualities: Perfect, Augmented, and Diminished.
Notice that Perfect Intervals do not have a minor quality.
In an earlier lesson we practiced these qualities using the unison. Let's see what this looks like with the interval of a 4th.
To review Unisons see Unisons and Seconds.
Reminder: a key signature is just a short hand for all the accidentals in a key. The easiest way to know if a note is part of a scale is to have your key signatures memorized.
To review Major Key Signatures see Major Key Signatures.
The bottom note is an A. Notice how the top note, D, is a note in the A major scale. In other words, there is no D♯ in the key signature for the key of A. Therefore, this is a Perfect 4th (P4). (A perfect 4th is also the distance of 5 half-steps.)
Notice how the top note, D♯, is a half-step higher than the note in the A major scale, D (natural). In other words, there is a no D♯ in the key signature for the key of A and the D♯ is one half-step higher than the D natural in the key. Therefore, this is an Augmented 4th (A4). (An augmented 4th is the distance of 6 half-steps.)
Notice how the top note, D♭, is a half-step lower than the note in the A major scale, D (natural). In other words, there are no flats in the key signature for the key of A and therefore D♭ is one half-step lower than the D natural in the key. Therefore, this is a diminished 4th (d4). (A diminished 4th is the distance of 4 half-steps.)