An eleventh chord is created by adding an 11th degree to any dominant 9th chord. For example, a C11 chord contains the notes C, E, G, Bb, D and F (1, 3, 5, b7, 9 and 11). The 3rd degree is usually omitted because of its clash with the 11th degree. Here are some common open 11th chord shapes.
C11
Db11
D11
Eb11
E11
F11
F#11
G11
Ab11
A11
Bb11
B11