A Perfect Fifth is the distance between the first and 5th notes of the scale.
|Perfect 5th – Up||
|Perfect 5th – Down||
|Perfect 5th – Chord||
The only exception is when the lower note is on the 7th (last) degree of the major scale.
This produces a so called “Diminished 5th” or “Flat 5” because it spans only 6 half steps instead of 7.