• When you have multiple voices, you have voices which move from one to the other.
• If one of the voice suspends, ie, stays in the same chord while the other voices all move, it leads to a "lag", called a suspension.
• A sus chord mimics this.
• First, setup a pitch that is going to be suspended (eg. GEGC). Let's suspend the C.
• The harmony moves to a different chord, say the V. Typically, this would be GBDF.
• But we are suspending, so let's play GDFC.
• It's interval above the bass is a 4th (G-C).
• Finally, we resolve the suspended note into a D.
• In figured bass, we write this as $V_{4-3}$. The resolved pitch, $D$, is a resolution.
• A good place to put a suspension is where a voice moves down.
• This is where we get the sus4 chords from.

§ Rules

• Suspension on voice moving down
• Preparation, suspension, resolution is important. Suspension resolves a step down.
• Suspension should occur on a beat that is stronger than a resolution.

• Reference: Write a Musical Suspension in Three Easy Steps by Brian Kehlenbach