The relationship between the first note (the root or tonic) and the fifth note in a major scale. It is also the inversion of the perfect fourth. In a diatonic scale, each scale degree, or note, is given a number, with the root or tonic, which is defined as tonal center of the scale or Key. In the Key of C major, C is the Tonic or scale degree one. D is scale degree 2, or a 2nd, E would be scale degree 3, or a 3rd, in the key of C continued, F, is scale degree 4 or a perfect 4th, and G, would be scale degree 5, or the perfect 5th. 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 cents smaller. The perfect fifth is considered the most consonant interval outside of the unison and octave. The strings on violins, violas, and cellos are all tuned to perfect fifths.