Perfect Fourth

This is the term for the relationship between the first note (the root or tonic) and the fourth note in a major scale, found five semitones higher. 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. For example, 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, and F is scale degree 4, or the perfect 4th.