I'm trying to explain what sus chords are, to answer the question "What do I play when I see a "sus" chord in a chart?", and to explain how to introduce "sus" chords as a substitution.
The Who's "Pinball Wizard" (like many other rock gems of the era) starts with the chords Dsus ... D (repeat ). I'm not sure whether in fact it's actually in D, but it's pretty easy to play Dsus ... D on the top 4 strings of the guitar. The notes are (Dsus) D-A-D-G changing to (D) D-A-D-F#. In other words, Dsus (also known as Dsus4, by the way) is a triad of D with the 3rd (F#) replaced by the 4th (G).
In classical music, the dissonance caused by the presence of the 4th would only be considered polite if the note (G) was already present, and just hung around from the previous chord, a.k.a. "suspended". Which is where the name "sus" comes from.
Of course, in jazz, we don't have much truck with chords with such a meagre collection of notes as a triad, so we're concerned with sus7 chords, often extended to sus9 or sus13. But the basic principle is the same - the 3rd is replaced by the 4th. So:
* The sus chord, in jazz, is a sort of modified dominant 7th chord
* It doesn't have a 3rd, it has the 4th instead.
Improvisers soloing over a sus chord will be able to take advantage of the fact that changing the 3rd of the chord to the 4th doesn't change the scale implied by the chord. So whether you see D7 or D7sus, the obvious scale is D mixolydian (i.e. G major). The sus chord is possibly less susceptible to alteration, so whereas you might blast out diminished patterns or an altered scale over a D7, it might be better to stick to D mixolydian over a D7sus chord. The sus chord sounds inherently less corny, so it doesn't cry out for alteration like a normal dominant.
In jazz the function of a sus chord is commonly as a suspension of the II chord over a V bass. So a II-V-I sequence such as
Am7 D7 G could be replaced by:
Am7 D7sus G
All that needs to happen is for the bass player to play:
A D G the same as usual, and the guitarist/pianist to continue Am7 instead of changing to D7 - i.e. play:
Am7 Am7 G
So the Am7 is suspended over the changing bass.
The Am7 over D bass gives the notes D-A-C-E-G, which are the root, 5th, 7th, 9th and 4th of D, giving D9sus.
Am9 over D also works, as that adds the 13th (B).
This can be used to give an easy trick for voicing sus chords in a chart:
* If you see a sus chord in a chart it will often be preceded by the II chord, so you just carry on playing the same II chord.
You can also introduce sus chords into the chart by substitution. The chord player just plays the II chord where a V is notated. For a guitarist or pianist this is about the easiest type of substitution to play, and one of the most effective, giving a more modern sound. Also, it can't upset the horn players too much because the implied scale isn't changed. The bass player can also bring about a sus chord by playing V bass under a II chord. So:
* The sus chord is a II chord, with V in the bass
* Pianist / guitarist may decide to change a V chord into a sus (by playing II)
* Bassist may decide to change II into a sus (by playing V)
Horns can use the II over V bass idea by playing II patterns over a sus chord (e.g. over Dsus play Am7 arpeggio patterns), and can also do their own subtitution by playing II patterns over a V chord that isn't written as sus, which can sound very effective.
When you see a sus chord in a chart for a conventional standard, it is often a "written in" substitution of this type. Here are some examples of standards where a sus chord on V is preceded by II (from the New Real Book):
1. "Don't Go To Strangers" by Kent & Mason, bars 5-6
2. "Long Ago and Far Away" by Jerome Kern, bars 25-26
3. "Moonlight In Vermont" by Karl Suessdorf, last 2 bars.
4. "Witchcraft" by Cy Coleman, bars 5-6
However, in the mid 60s with tunes like Herbie Hancock's "Maiden Voyage", sus chords were used in a new way in parallel key changes without any kind of normal resolution. The sus chord lacks the tritone between 3rd and 7th, which gives it a more stable feel than a dominant 7th chord, so it doesn't really push towards resolution. The sus chords in this style of playing can still be voiced and played over in the same way.
The 3rd is sometimes added back into the voicing to create a more complex sound, usually higher than the 4th. So:
* The sus chord doesn't always function as a dominant
* The sus chord can contain the 4th and 3rd
Actually, in "Pinball Wizard" the Dsus functions as a tonic chord.
The sus chord is described in a variety of ways in charts. You might see D7sus, D7sus4, or D9sus, etc. Then there's D11, which implies it isn't a sus chord, as there's nothing to indicate that the 3rd isn't present, but it sounds pretty awful if the 3rd is below the 4th, so it's better treated as a sus chord with the 3rd added somewhere above the 4th. You also find the sus chord described in slash notation (chord / bass) - e.g. Am7/D, Em7/D, C/D.
There is a fairly common altered sus chord, susb9. I'll write about it some other time.