By adding another 3rd interval on top of a 9th chord, it is possible to create an 11th chord. Depending on the 3rd and 7th of the chord, you can create major, minor and dominant 11th chords. The chord shown in the next diagram is a C minor 11th (Cm11). By raising the 3rd of the chord from Eb to E@ it could be changed to a dominant 11th chord (C11). By raising the 3rd and the 7th it could be changed to a major 11th (CMaj11).
The more notes there are in a chord, the more difficult it becomes to play. When using extended chords, it is common to play only the guide tones as shown here. The right hand plays the 11th, b7th and b3rd of the Cm11 chord.
When playing Major or Dominant 11th chords, the 3rd and 11th ( = 4th) degrees have the potential to clash and sound bad as shown in the first and second bars of the following example. For this reason it is common to leave out the 3rd degree when playing 11th chords, as shown in the final three bars of the example. The C11 chord without the 3rd is created here by using a Gm7 chord over a C bass note.