So a key can start on any note. It can also start any place on the keyboard. So you can play a melody in C-Major at either end of the piano, and it will sound identical, except it will be in either a higher or lower register.

Ah, but there's more! There are also Minor keys, which can also start at any point in the octave. What makes a minor key different is the spacing scheme. The Natural Minor is W H W W H W W. There is also a Harmonic Minor (W H W W H W+H H) and a Melodic Minor, which can appear in both an Ascending (W H W W W W H) and Descending (W W H W W H W) forms.

And there are many other keys, some peculiar to other cultures, or other musical genres. There are Blues keys, folk keys, rock keys, jazz keys But the Major and Minor keys form the bulk of most Western music. The whole ideas of playing these subsets of the octave is that melodies sound subtly different in different keys, that there are different tensions and resolutions. Sometimes composers even change keys in the middle of a melody.

They say music "is all math". But its not. Math makes sense.

Comments are welcome.