11th & 13th

The upper extensions

Prove it — just do the practice exercise.
C → F 17 semitones
C (root) F (11th)
11th = octave + 4th = 17 semitones
→ In m11: modal, Dorian character
→ #11: Lydian sound (Maj7#11)

13th = octave + 6th = 21 semitones
→ In 13: full big band sound
→ Color without need to resolve

These intervals are only used in extended chords. They add "flavor" — not the basic function.