The musical interval of a perfect fifth is the relationship between the first note (the root or tonic) and the fifth note in a major scale. It is the inversion of the perfect fourth.
It can be produced by starting on a high note and playing the fifth below or by starting on a low note and playing the fifth above.
A perfect fifth in just intonation corresponds to a pitch ratio of 2:3 or 1:1.5 while in an equal tempered tuning, a perfect fifth is equal to seven semitones, a ratio of 1:27/12 (approximately 1.4983), or 700 cents,two centss smaller.
The strings on violins, violas, and cellos are all tuned to perfect fifths.