Originally Posted by: Lao_Tzuhi everyone I have just worked out the modal approach to things but I would like to understand where sus chords fit in because they are neither major or minor since the 3rd is replaced by either the p4 or m2.

Congrats. :)

Now, you are ready to start thinking in terms of tonal in addition to modal.

The tonal approach (as in tonality) deals with Functional Harmony. As in, "What is the function of such and such a tone (note), sequence of tones, or chord, or sequence of chords, etc.?"

In the case of suspended chords, think about their name: why are they called "suspended"?

The reason is because they "suspend" or temporarily delay the arrival of the major or minor third. The suspended note is a chord tone held over or "suspended" from the previous chord, leaving you hanging waiting for the arrival of the major or minor third to give a resolution. The classic, timeless example is any Bach chorale in which he uses the candential I 6-4 chord or the V7 chord or even the I at the end of a line in a suspended form, then follows with the third. Example with a V7 4-3 - I cadence:

E |------|------|------|------|
B |-3--5-|-3--3-|-3--3-|-2--3-|
G |-2--2-|-2--0-|-1--0-|-0--2-|
D |-4--5-|-4--0-|-3--2-|-2--4-|
A |-5--4-|-3--2-|-2--0-|-0--5-|
E |------|------|------|------|

List of chords and function:
D (I) - A7 (V7) | D (V7 of IV) - G (IV) | G#dim7 (vii dim7 of V) - A7sus4 | A7 (V7) - D (I)

The D note in the G#dim7 chord is held over to the A7 creating an A7sus4 chord. Which is then resolved to an A7 which then resolves to the D major. Voice leading!

This is similar to jazz (classic jazz standards, not "smooth jazz" or "jazzy sounding" fusion), except there are rarely straightforward major, minor and dominant 7 chords. There are instead mostly extended harmony chords like major 7ths, minor 7ths, 6-9, etc. But the concept is exactly the same: there is purpose and function to the harmony (voice leading!).

Examples can also be found in pop and rock guitar, like Van Halen tunes like "Unchained", Beatles tunes like "I Need You", etc. Some of the suspended chords in those tunes are tonal uses, most of them are simply ornamental (or modal).

Now, of course, you can simply use them for ornamental purposes: because you like the sound of any given suspended chord at any given place in the music. But this is precisely a modal conception, and why it is different from a tonal conception.

Both are useful tools to have in your mind. :)