The Perfect 4th Interval (P4)
An interval is the distance between two notes, counted in semitones. Each of the thirteen simple intervals has its own page: reference songs that open with it, where it sits on the guitar fretboard, its inversion, and the chords built from it.
- The Unison (P1) — 0 semitones
- The Minor 2nd (m2) — 1 semitone
- The Major 2nd (M2) — 2 semitones
- The Minor 3rd (m3) — 3 semitones
- The Major 3rd (M3) — 4 semitones
- The Perfect 4th (P4) — 5 semitones
- The Tritone (TT) — 6 semitones
- The Perfect 5th (P5) — 7 semitones
- The Minor 6th (m6) — 8 semitones
- The Major 6th (M6) — 9 semitones
- The Minor 7th (m7) — 10 semitones
- The Major 7th (M7) — 11 semitones
- The Octave (P8) — 12 semitones
The perfect 4th spans 5 semitones — open and sturdy — the interval between adjacent guitar strings. Measured up from a major-key tonic it lands on the 4th, so it is diatonic to every major key. Inverted (the lower note moved up an octave) it becomes the perfect 5th.
The classic way to internalize it: sing a song that opens with it — Amazing Grace (the opening leap on "A-maz-"); Here Comes the Bride (the first two notes); Eine kleine Nachtmusik (the opening G–D call). You also hear the perfect 4th inside chords built on it, like Sus4, 7sus4, 11, m11. Then drill it on the ear trainer until you name it without thinking.