eli5: in music, since there are 12 notes per octave, why are only 7 letters used instead of 12?

Question

eli5: in music, since there are 12 notes per octave, why are only 7 letters used instead of 12?

In: 15

Answer 1

There are only 7 different notes, so 7 letters : A B C D E F G. An octave goes from one note to the next same note : A B C D E F G A, so 8 notes and not 12.

I don’t know where that 12 comes from.