What makes a serve legal?

Under Law 9, a correct service needs all of the following:

• No undue delay once server and receiver are ready. Pausing after the backswing is complete counts as undue delay.
• Server and receiver stand within diagonally opposite service courts, without touching the boundary lines.
• Some part of both feet of the server and receiver stays in contact with the court, in a stationary position, from the start of the service until it is delivered.
• The server's racket first hits the base of the shuttle.
• The whole shuttle is below 1.15 metres from the court surface at the moment it is hit.
• The racket keeps moving forward continuously from the start of the service until the shuttle is hit.
• The shuttle travels upwards off the racket, passes over the net and would land in the receiver's service court.
• If the server attempts to serve and misses the shuttle, it is a fault.

Service faults with the net: it is also a fault if the served shuttle lands on top of the net and stays there, crosses the net and is caught in it, or is hit by the receiver's partner.

The 1.15 m fixed height has been mandatory at high-level tournaments since December 2018 and is the only service-height rule in the current Laws.