All chords are constructed from the Major scale. The extended C Major scale is shown next, with the each scale degree indicated.
The major chord is constructed from the 1st, 3rd and 5th notes of the major scale.
If you add the flattened seventh note to the major chord, a dominant seventh chord is formed. This type of chord is often abbreviated to 'seventh' and written as such.
To form a ninth chord, add the 9th degree of the major scale to the seventh chord.
To form the eleventh chord, add the 11th degree of the major scale to the ninth chord.
To form the thirteenth chord, add the 13th degree of the major scale to the eleventh chord.