13ths

Technically, a 13th chord is an 11th chod with the note a 13th above the root added (octave plus major 6th).

It is most often played as a dom 7th chord with the 13th added.

eg.

C13

LH C
Rh: Bb E A


A major chord:

Cmaj 13

LH C
RH B E A