The exact regulation for sending the shuttle diagonally over the net to an opponent falls under the service rules of badminton.
It's often misunderstood by players, but only three visible court lines are used during the serve and apply to the specific rules for serving.
Similar to the rules in tennis, the service court markings are not actually related to individual rallies.
Hence, a rally begins (and continues) after the server sends the shuttlecock crosscourt over the net towards the receiver.
The aim of some special restrictions in the badminton serve is to avert the server from winning an unfair advantage.
For example: Unlike tennis games, the server must use an underarm hitting style to send the shuttle upwards and over the net. Furthermore, only one receiver can return the service when playing doubles games.
In fact, the official rules and regulations of badminton are clear on the permitted technique for serving. As such, servers must strike the shuttle below a fixed height of 1.15 metres (e.g. around the same level as their lower rib cage).
We can simplify this definition. The continuous forward movement takes place from a general area defined as the height below the waist - but above the player's shorts.
Even though the serving action is the same for all players, the two key differences between singles and doubles service rules are:
There are two (2) service boxes inside each half of the singles court. One back box is for the left service and the other for the right service court.
The same is true for a doubles game. But, the service boxes are wider and the doubles back line makes them a little shorter in length.
A service box in badminton is only used during a serve. Each player must serve from inside the box markings.
Players serve 'cork first' (not feather) and crosscourt over the net to the receiver, who will receive it from inside the opposite service box.
The shuttle should be traveling in a diagonal direction across the playing area. Thus, the actual flight should be up and over the net towards your opponent's service court back box.
Players must remain inside their own service box, and have both feet in contact with the floor, until the receiver strikes the badminton shuttle and completes the return.
Even so, if the shuttlecock (also called a birdie) touches the ground outside the box lines, and in play, the server would win the point.
BWF codified badminton rules govern competitive events and tournaments. The regulations allow five different formatted types of male and female opponents.
In most cases, badminton gets played as an indoor court game. Its competitors will be either two single players (singles) or two pairs of players (doubles).
The rules for badminton doubles have slight variances to the singles matches. For example, a game of doubles would have four players - two against two.
Either of the paired players may take a shot in the rallies, following the initial serve. Hence, taking turns is not required.
Note: The short video should help you clear up any arguments you might be having with an opponent about their serve being an illegal one.
Service Rules in Badminton: Players Serving in the Serve Area