The fifth is the second most significant interval after the octave. We hear open fifths in the opening of Richard Strauss's Also sprach Zarathustra (the music from the film 2001: A Space Odyssey) — these wide, open intervals evoke the vastness of the universe.
Violin, viola, and cello are tuned in fifths (the double bass in fourths). Parallel fifths were forbidden in Renaissance polyphony — they locked voices together and robbed polyphony of its independence. In medieval organum, by contrast, fifths were the core harmonic unit.
A chain of perfect fifths forms the circle of fifths — the fundamental scheme that orders all 24 keys and vividly shows how closely they are related. This is why the fifth is regarded as the “constructive” interval of harmony. The diminished fifth (the tritone), by contrast, was for centuries reputed the most tense, “devilish” interval and demanded an obligatory resolution.
