In doubles if the serving side wins a rally the same

In doubles, if the serving side wins a rally, the same player continues to
serve, but he/she changes service courts so that she/he serves to a different
opponent each time. If the opponents win the rally and their new score is
even, the player in the right service court serves; if odd, the player in the left
service court serves. The players' service courts are determined by their
positions at the start of the previous rally, not by where they were standing
at the end of the rally. A consequence of this system is that each time a side
regains the service, the server will be the player who did not serve last time.
Scoring
Main article: Scoring system development of badminton
Each game is played to 21 points, with players scoring a point whenever they
win a rally regardless of whether they served
[13]
(this differs from the old
system where players could only win a point on their serve and each game
was played to 15 points). A match is the best of three games.
If the score reaches 20-all, then the game continues until one side gains a
two-point lead (such as 24–22), except when there is a tie at 29-all, in which
the game goes to a golden point. Whoever scores this point will win.
At the start of a match, the shuttlecock is cast and the side towards which
the shuttlecock is pointing serves first. Alternatively, a coin may be tossed,
with the winners choosing whether to serve or receive first, or choosing
which end of the court to occupy first, and their opponents making the
leftover the remaining choice.
In subsequent games, the winners of the previous game serve first. Matches
are best out of three: a player or pair must win two games (of 21 points
each) to win the match. For the first rally of any doubles game, the serving
pair may decide who serves and the receiving pair may decide who receives.
The players change ends at the start of the second game; if the match
reaches a third game, they change ends both at the start of the game and
when the leading player's or pair's score reaches 11 points.
Lets
If a let is called, the rally is stopped and replayed with no change to the
score. Lets may occur because of some unexpected disturbance such as a
shuttlecock landing on a court (having been hit there by players playing in

adjacent court) or in small halls the shuttle may touch an overhead rail which
can be classed as a let.
If the receiver is not ready when the service is delivered, a let shall be called;
yet, if the receiver attempts to return the shuttlecock, the receiver shall be
judged to have been ready.
Equipment
Badminton racquets
Badminton rules restrict the design and size of racquets and shuttlecocks.
Racquets
Badminton racquets are lightweight, with top quality racquets weighing
between 70 and 95 grams (2.5 and 3.4 ounces) not including grip or strings.