The musical interval equivalent to seven half-tones or 3.5 whole-tones. Two notes played a perfect fifth apart will sound very stable, due to the ratio between their frequencies (2:3). The only more stable interval is the octave. Chords consisting only of a perfect fifth and an octave are very popular in modern rock and are referred to as power chords.

A chord built only with a note, a third above it, and its perfect fifth I have always found to be boring. It is too stable.

Called a triad, major if the third is composed of four semitones, and minor if the third is composed of three semitones.

For my own playing--improvisation actually--I prefer the instability of an added seventh above the root, a minor seventh is OK, but a major seventh is far more interesting to my ear.

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