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The primary structure in which we organize notes is the scale. The interval relationships among a series of notes determine the specific type of scale. There are major, minor, modal, diminished, whole tone, pentatonic and chromatic scales. Though other scales exist, these basic scales form the foundation of most music. Each of these scales contains a unique combination of intervals that distinguishes it from all the others.

Building a Scale with Intervals

If you look at the note Middle C on the piano, you will see that the note immediately below it is a B. The distance between middle C and this B is one half-step. This is also known as a minor 2nd:  

The next note immediately above that Middle C is a C#. The distance between Middle C and that C# is also a half-step (or minor 2nd):

As you see, there is no smaller distance you could travel from that middle C than the half-step downward to B or upward to C#.

The next interval in size is called the whole step. This is also known as a Major 2nd. The whole step simply covers the same distance as two half-steps. Go back to Middle C. From this note, go up to the D right above it. This is the interval of a whole step.

Notice, you had to pass the C# to get to the D. If the distance from Middle C to C# was a half-step, then the distance from C# to the D above it is also a half-step. These two half-steps, put together, make a whole step.