13.15What is a 13th?

13 = scale degree 6.

For G:

G major scale:

G A B C D E F♯

13 = E.

But for a dominant 13 chord, the base chord is G7:

G B D F

Add extensions:

9 = A

11 = C

13 = E

The theoretical full chord:

G B D F A C E

But again, we usually omit notes.