1.3The most important keyboard geometry

Look at the white keys:

C D E F G A B C

Most adjacent white notes are separated by a whole tone:

C → D = whole tone

D → E = whole tone

But there are two exceptions:

E → F = semitone

B → C = semitone

No black key exists between them.

This fact controls an enormous amount of music theory.

Memorize:

E-F and B-C are natural semitones.

Or:

ми-фа and си-до are half steps.

Everything else between adjacent natural notes has a black key between them.