Musical Intervals: How They Are Measured
The Diatonic Major scale is the most important scale in our Western musical system and has become the scale by which all else is measured. The spaces between its notes are like inches and it is THE standard used to measure harmonic distances between notes. The distance between notes is called an interval.
The major scale is based on a specific arrangement of intervals creating a pattern that only exists in the major scale. A whole step is also known as a Major 2nd interval. A half-step is also known as a Minor 2nd. Like stepping stone on a path, there are seven steps (intervals) that lead one from a root note to its octave equivalent. The intervals that create a major scale are as follows:
When this arrangement of intervals is overlaid onto all the possible notes that exist within the C to C octave, it creates the C Major Scale, C,D,E,F,G,A,B. This same arrangement of intervals can begin on any note. However, it always produces a major scale. This scale is more accurately called the Diatonic Scale but you will see both terms used to describe this specific arrangement of intervals.
Since the same interval structure creates all major scales, numbers can replace the lettered notes to create a universal system of "measuring" any interval. This universal system is what is used to describe the distance between any two notes. It is essentially a musical yardstick. Whether you are analyzing the notes of a chord or another scale (and there are plenty), the arrangement of those notes is always compared to the Diatonic Scale.