The legal serving motion
The serve must be underhand — the paddle moves in an upward arc — with contact made below waist height and the paddle head below the level of the wrist at the moment of contact. This rule exists to keep the serve a controlled, defensive shot rather than an overhead weapon. For how to build a repeatable, deep serve within these rules, see serve technique and power.
Foot position and faults
At the moment of contact, the server must be standing behind the baseline, and at least one foot must not be touching the ground on or in front of the baseline, or outside the imaginary extension of the sideline. Stepping fully onto or over the baseline during the serving motion before contact is a foot fault.

Where the serve must land
The serve must clear the net and the kitchen entirely, landing in the diagonally opposite service box. A serve that lands in the kitchen, even by a fraction, is a fault. This is one reason depth control matters as much as raw pace — see serve technique and power for the depth-first approach to serving.
Let serves
If a serve clips the top of the net and still lands in the correct service box, it's called a let and is replayed without penalty. If it clips the net and lands out or in the kitchen, it's a fault, just like any other out serve.

