·         We've seen how most of the important jazz chords can be formed by sharpening and flattening the notes of the major triad I III V in different combinations.  The only remaining alteration to the triad that we haven't considered is sharpening the third.

·         Check what happens when the third is sharpened - it becomes the fourth (confusing, isn't it?).  This produces the suspended fourth, symbol sus or sus4. 

·         Example [6.1] is BbΔ sus4, shell voicing.  You might think it sounds like nothing on earth - but try moving the suspended note (Eb) down to the third (D) (Example [6.2]).  Do you feel the sense of release?  That's why it's called a suspension!  The pressure to resolve the Δsus4 to the straight Δ is irresistible.

·         Example [6.3] is Bb-7 sus4, or it could be Bb7 sus4.  It all depends on context.  Play Example [6.3] followed by straight Bb7.  The sense of release is there, because of the semitone movement from Eb to D.  But the same resolution doesn't occur if you play Example [6.3] followed by Bb-7, because the whole tone from Eb to Db is a less tense interval.

·         In fact it's unlikely you'll ever come across the symbol -7 sus4.  It's usual name would be -11, and it's a perfectly good extended minor chord, needing no further resolution.

·         On the other hand the dominant seventh sus4 is such an ambiguous customer that you can leave it unresolved (you could call it a 7 11 chord).  There are entire tunes based on this ambiguity.  The A section of the great "Miles" (sometimes called “Milestones” from the album of the same name) by Miles Davis is based on an unresolved C7 sus4 (Example [6.4]).

·         "Miles" is an example of modal jazz, and 7 sus4 chords, because of their mysterious, unresolved nature, are specially common in this kind of music.