So, to understand Musical Intervals, we need to find a better measurement system than just the number of notes in the interval. With a little help from a piano keyboard we can shed some light on this mystery.

If we play the white keys in sequence from C to the next C we'll have a C Major scale, a sound we all know and love, but if we play in sequence all the keys, both the white and the black, we'll listen to all the notes used in western music. This is called a Chromatic scale. The interval between two consecutive keys (black or white) is the smallest we use in the west, while in some Eastern and Arabic countries they use even smaller ones. We will call this interval a Half Tone (also called a Half Step or Semitone.

Now we can clearly see why C-D is quite different from E-F. There are no black keys between E and F so their interval is a half tone, while between C and D we find a black key so the interval between them is made by 2 half tones or a Whole Tone (we'll call it a Tone, for short).

We said earlier that C-E and D-F were both Third intervals, but they sounded different. If we count the number of Half Tones beetween them we see that C-E is equal to 2 Tones (4 Half Tones) while D-F is 1 and a Half Tone. To have the same interval as C-E we should add one more half tone and use one of the black keys: D-F#.

So even though the number of notes on the Major scale (the white keys) gives the name to the interval (Third, Fifth, Seventh etc.), it's the number of Half Tones that really defines an interval.

We will learn to give a name to each Musical Interval on the next page.