Are there rules about when a note is a 4th or 11th? or 6th and 13th?
It would make sense that an 11th and 13th should be an octave about the root but in many chord formations I see the 11th or 13th near the root. Why in that case do they not become 4ths and 6ths?
Extensions such as 9/11/13 can be voiced anywhere in the chord. Generally they will be in the top half of the chord but they certainly don’t need to be an octave above the root.
Ultimately, the 2 is the 9, the 11 is the 4, and the 13 is the 6, but thinking of them in terms of extensions (past the 7th) is simply more effective and practical.
For example if you are reading a lead sheet, you will frequently see symbols asking for extended chords such as:
If you memorise the scale degrees as 9/11/13 - you will find it much easier to remember and find these voicings.
Also, jazz nomenclature rarely uses 2 and 4. There are some instances such as ‘add2’ chords but again these are rare. There is the sus4 chord too which is more common but 9/11/13 chords are much more common in jazz and so it makes sense to learn, memorise, and refer to these tones as upper extensions.
An important point with the “13” or “6”:
The 13 is slightly different… for example, if you have the 9 and 13 in the chord but no 7th it becomes a 6/9 chord.
If the major 7th or b7th is not in the chord, the 13 is always referred to as the 6 - this is an important rule to remember!
The 6/9 is a common chord and you will see it used in many of PianoGroove jazz standard lessons. A voicing for C69 would be C-G in the left hand and E-G-A-D in the right hand. Notice there is no ‘B’ in the chord which is the 7th and so the 13 is now referred to as the 6.
What about the 9 or 11 with no 7th in the chord?..
It’s rare that you would have the #11 in the chord without the major or dominant 7th. There are some examples such as the slash chord Dmaj played over a Cmaj ( IImaj / Imaj ) which can give you a major #11 sound but again its usage is quite rare.
It’s also unusual to have the 2 in the chord without the 7. If it was it can be called an ‘add9’ for example Cadd9 which would be a C Major triad with the 9 or 2 voiced in the chord.
Complicated I know! It gets much easier with time.