Sus Chords on the Piano

Suspension chords are chord which are put before dominant chords most often in order suspense the clear tension of the dominant resolving to the tonic.
What we have here is actually a clear case of a subdominant.

The first suspension chord we're going to discuss is the sus7 chord.

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Sus7 chords appear often before the dominant seven as I've just mentioned.
Let's look at Gsus7 for example.

This chord contains three notes, the root note of the fifth degree (G) the fourth note of the chord (C) and the seventh note (F).
This chord is almost similar to G7 accept is has the fourth note (C) instead of the third note (B).

Although sus chords look almost like dominant chords their duty in music is to suspense the tension before leading to the dominant. They function as a subdominant really.

Sus chords do not have the identity of a major chord or a minor chord since they don't have the third in the chord (Because the fourth note is being solved to the third note of the next chord). They basically have three notes; the root note, the fourth note and the seventh note.

Here's an example of Gsus suspending the tension before going to G7, the dominant (V7) of C.

The fourth note C will is functioning as suspension by creating a dissonance and is about to resolve by step in the following chord (G7) to the third note of the chord (B).
That's the true meaning of the suspension chords.

Now you may wonder what the function of the seventh in the chord is.

Well the seventh note (F) in our case is being held to the next chord G7 and resolves in the tonic to the last chord to a point of a final rest.

Sus4 Chords

Let's see another case of a sus chord with a bit less tension; the sus4 chord.
The sus4 chord functionality is the same only it doesn't have the seventh note but the fifth instead which is more neutral then the seventh.