fraserhutch said:
I disagree - the 9th is always implied, and is not optional. The ONLY time you specify the 9th is when it is altered, as in a G13 b9.
Well this is somewhat correct. In extended chords, like 9th, 11th, and 13th chords, all the extensions up to that point are always implied.
9th = 1 3 5 7 9
11th = 1 3 5 7 9 11
13th = 1 3 5 7 9 11 13
Now, obviously, all of these notes aren't always played. And the 11th is probably omitted more than it is played (unless it's raised a half step). But the very nature of extended chords is the continued stacking of 3rds, so technically a full 13th chord will always have a 9th and 11th. But the 9th and 11th ARE optional. The 13th and b7th are not optional in a 13th chord, because without either of them, it wouldn't be a 13th chord anymore.
For instance, a common voicing on guitar for A13 might be:
--------
---7----
---6----
---5----
--------
---5----
This is, from low to high: root-b7th-3rd-13th. There's no 9th, but it's still A13 because it has a b7th and a 13th. But if you played this chord:
---7----
---7----
---6----
---5----
--------
---5----
which is, low to high: root-b7th-3rd-13th-9th, it's still just called A13. The only time the 9th is mentioned is a 13th chord name is when, as fraser pointed out, the 9th is altered.