When playing 11th chords, a particular problem arises because the 11th degree is the same as the 4th and this clashes with the 3rd of the chord which is only a semitone distant.
This it particularly noticeable when the chord played by a harmonic instrument is in the same register as the melody player’s 11th.
When playing suspended triads and seventh chords, the 4th replaces the 3rd, so there is no clash. However, when playing 11th or 13th chords, the 11th degree is either sharpened by a semitone, or omitted from the chord. In minor chords, the 11th does not need altering, because it is already a whole tone away from the flattened 3rd degree and therefore doesn’t clash as much.The following example demonstrates the sound of major and dominant 13#11 chords.
Here is an improvised line created from the notes of these two chord types.
Now try this progression which uses #11 chords amongst other chord types.