13.11What is an 11th?

Take a seventh chord and add:

9 and 11.

For C:

11 = F.

Theoretical Cmaj11:

C E G B D F

But there is a problem.

Play:

E + F

right next to each other.

That minor second can be quite harsh.

So natural 11 over a major-third chord often creates a conflict between:

3 and 11

For C:

E and F.