Perfect Intervals
The stable and fundamental pillars of harmony.
The Perfect Group
Unisons (1), Fourths (4), Fifths (5), and Octaves (8) are classically defined as 'Perfect'. They have a hollow, open, and stable sound, occurring naturally in the harmonic series.
Summary
Remember the Perfect intervals and their exact semitone distances: Perfect Unison (1) has 0 semitones, Perfect 4th (4) has 5 semitones, Perfect 5th (5) has 7 semitones, and Perfect Octave (8) has 12 semitones.