Perfect Fifth

Play C and G together. You hear a root and its fifth, but there is no E or E flat to make the sound clearly major or minor.

Add E and you get C major. Replace E with E flat and you get C minor. C and G remain in both chords. The third supplies the clearest major or minor color, while the fifth reinforces the root and gives the chord a firm outer frame.

This is what musicians mean when they call the perfect fifth stable or open. The words describe a concrete relationship: the notes blend strongly, and the interval leaves room for another note to define the chord quality.

Count the letter names from C to G, including both ends: C, D, E, F, G. That makes it a fifth. Count the piano steps and you find seven semitones.

The same method works from any starting note. D to A is five letter names and seven semitones, so it is another perfect fifth. C to G flat still spans five letter names, but it covers only six semitones. That smaller interval is a diminished fifth, one spelling of the tritone.

If you need the full counting method, start with What Is an Interval?.

Unisons, fourths, fifths and octaves use the quality name perfect. Seconds, thirds, sixths and sevenths use major or minor.

The labels tell you which family an interval belongs to. A perfect fifth can be made one semitone smaller or larger, but the results are called diminished and augmented rather than minor and major.

Inversion keeps the family intact. Move the lower C of C to G above the G and the new interval is G to C, a perfect fourth. Perfect intervals invert to perfect intervals. Major intervals invert to minor ones, and minor intervals invert to major ones.

The word perfect comes from older theory and tuning traditions, so inversion is a useful property of this interval group, while the name itself comes from older theory and tuning traditions.

A perfect fifth above the root appears in many common chords:

Some chords change the fifth because that change is central to their sound. A diminished chord lowers it; an augmented chord raises it. Other chord voicings may simply omit the fifth, especially when the root and third already make the harmony clear.

This is why the fifth can seem less noticeable than the third or seventh. It is present so often that your ear treats it as support until a chord alters or removes it.

A power chord such as C5 contains C and G, often with another C on top. Because it has no third, it does not commit to C major or C minor by itself.

The simple root-fifth relationship also remains clear with amplification and distortion. The overtones of the two notes overlap in a regular way, so the result can sound full without needing a complete triad.

Context can still make a power chord feel major or minor. A melody, bass line or surrounding scale may supply E or E flat even when the guitar voicing does not. The power chord leaves that information unstated; the rest of the music can fill it in.

Move upward by perfect fifths and the sequence begins C, G, D, A, E and B. Each new major key in that direction adds one sharp. Moving the other way through fifths removes sharps and then adds flats.

That pattern is the basis of the circle of fifths. It helps you see which keys share many notes, which chords commonly follow one another and why dominant-to-tonic motion is so common.

After twelve equal-tempered fifths you reach B sharp, which uses the same piano key as C. You have also climbed the same total distance as seven octaves, so the pattern closes on a modern piano. The spelling records the path through the keys even though the final key sounds like C.

A pitched note contains a fundamental and quieter frequencies above it called harmonics. In the harmonic series, the third harmonic has a frequency three times the fundamental. Bring it down by one octave and its relationship to the fundamental is 3:2, the ratio of a pure perfect fifth.

For example, if one note vibrates 200 times per second, a pure fifth above it vibrates 300 times. Their wave patterns line up regularly, and several of their harmonics reinforce one another. This helps explain why the interval often fuses into one broad sound.

A modern piano uses equal temperament. Its perfect fifth measures 700 cents, while a pure 3:2 fifth is about 702 cents. The difference is small but useful: making every fifth slightly narrower allows twelve fifths to meet seven octaves exactly.

Twelve pure fifths would overshoot those seven octaves by about 23.5 cents. That gap is the Pythagorean comma. Tuning and Temperament explains how different systems distribute this mismatch.