“6” versus “13” rule: label the 6th of a chord “6” only if the 7th is not present; if the 7th is present, label the 6th as “13”
“11” versus “sus” (“sus” means “4”): label the 4th of a chord “sus” only if the 3rd is not present; if the 3rd is present, label the 4th an “11” (C7sus has C–F–G–B♭). (There is an exception to this in more advanced jazz theory, which is not within the purview of this text.)
♭5 versus ♯11: If the chord has a normal 5th (C–E–G–B♭–F♯), call the F♯ a “♯11” (because you won’t have a chord with a regular and altered 5th at the same time). Otherwise, you can call the F♯ of C–E–B♭–F♯ a “♯11” or a “♭5.”
