The white notes scale
If we now fill up the staff notation system we learnt (still fresh in our heads) with notes, i.e. one note on every line and in every interval, continuing above and below with ledger lines, we get the white notes scale. This is exactly the same as the white keys on a piano.
As we can see here in the example, the following note names repeat continuously:
C - D - E - F - G - A - B.
So it's important to look at these closer, as they form the basis of our staff notation system. If we consider the above staff notation alone, we could perhaps claim that all notes are equi-distant to one another. But appearances are deceptive, and this becomes clear when we take a closer look at a piano keyboard.
And there you see it; there isn't a black key between every white key.
There is no black key between E and F and B and C. The space between our white notes is obviously not the same everywhere.
At this point I would like to introduce the terms "half tone" and "half tone step". A half tone step is the smallest tone unit used in music.
As we can see, E and F are directly beside one another; they have a half note interval. We call the interval from E to F a half tone step.
B and C are exactly the same. This is also a half tone step.
But what is the situation with C and D, for example? The interval from C to the next black key is a half tone step, and from the black key to D is also a half tone step.
So 1/2 + 1/2 = 1, two half tone steps counted together produce a whole step (or a whole tone).
