To kick off this course we answer the first basic question... 'Where does the 9th come from if the scale only has 7 notes?' As with most theoretical questions, the answer is simple but only if you know it!
When we are dealing with 9th, 11th or 13th chords we need to extend our basic major scale to two octaves. That is how we find those notes. Have a look at this diagram:
As you can see we have extended the scale by a second octave, which are exactly the same notes but all higher. It suddenly because clear therefore that the 9th note is identical to the 2nd note. Calling it a 9th therefore, helps us realise that the note is typically higher in the chord, and needs to include notes from the previous octave as well as the ninth. So, get the two octave C major scale drawn out and we'll go start the first chord in the next lesson.