Badminton draws two courts on one floor, and swaps between them mid-rally

A badminton court carries more lines than any other racquet court, and the reason is that it is two courts. The Laws are explicit that one marked rectangle serves both formats: the court as drawn in Diagram A can be used for both singles and doubles play.

Reference That the court as shown in Diagram A can be used for both singles and doubles play, and that all lines are 40 mm wide and form part of the area they define, from Law 1.

Two details in that sentence carry more weight than they look. The lines are 40 mm wide — which, on a court whose full diagonal is 14.723 m, is not negligible. And every line forms part of the area it defines, so a shuttle landing on a line is in.

The posts do not move

The most economical decision in the whole design is where the posts go.

The posts shall be placed on the doubles side lines, irrespective of whether singles or doubles is being played, and the posts or their supports shall not extend into the court beyond the side lines.

Reference That the posts are placed on the doubles side lines irrespective of whether singles or doubles is being played, and may not extend into the court beyond the side lines, from Law 1.

Tennis, faced with the same problem, does the opposite: a doubles net used for singles must be propped up at the singles sideline by two singles sticks, so that the barrier is the correct height where the singles court ends. Badminton does not bother, and can afford not to, because its net is nearly level — 1.524 m at the centre and 1.55 m over the doubles sidelines, a difference of 26 mm.

Reference The net heights of 1.524 m at the centre of the court and 1.55 m over the side lines for doubles, and the post height of 1.55 m, from Law 1.

Twenty-six millimetres across a net at least 6.1 m wide. Tennis, over a doubles court of 10.97 m, drops 156 mm and mandates a strap to produce it. One sport treats the sag as a design feature; the other treats it as an error to be kept small.

Where the two courts disagree

The boundaries used for singles and doubles differ, and the differences do not run the same way.

In doubles the court is wider — the side boundary is the outer sideline rather than the inner one. But the service court in doubles is shorter: the serve must land inside a back service line drawn well inside the rear boundary. In singles the court is narrower and the service court runs all the way to the back.

The result is that a doubles serve must be delivered into a box that is wide and shallow, and a singles serve into one that is narrow and deep. The two formats do not simply use more or less of the same court; they use differently shaped parts of it, and the shapes are almost complementary.

That is the geometric reason the two games look so unlike each other from the first shot. A shallow service box makes a low serve to the front the standard opening in doubles, because there is little depth to attack. A deep, narrow one makes a high serve to the back standard in singles, because there is depth to exploit and little width to be beaten by.

The serve belongs to the score, not to the player

The other half of the design is who serves from where, and badminton answers it with the scoreboard rather than with a rota.

In singles, the players serve from and receive in their respective right service courts when the server has not scored or has scored an even number of points, and their left service courts when the server has scored an odd number.

Reference The singles rule that service is from the right service court on zero or an even score and from the left on an odd score, from Law 10.1.

The same principle governs doubles: a player of the serving side serves from the right service court when the side has not scored or has an even score, and from the left when the score is odd.

Reference The doubles rule that service is from the right service court on zero or an even score and from the left on an odd score, from Law 11.1.

An observer who knows the score knows which box the serve must come from, and an observer who sees the serve knows something about the score. The two are the same information.

Doubles then adds the rule that makes partnerships intelligible: the player of the receiving side who served last shall stay in the same service court from where they served last, and the reverse pattern applies to their partner. Players do not change their service courts until they win a point while serving.

Reference That the player of the receiving side who served last stays in the same service court, that the reverse pattern applies to their partner, and that players do not change service courts until they win a point when serving, from Law 11.1.

So a doubles pair swaps sides only on winning a point on their own serve. Losing the rally changes nothing about where they stand.

What happens when it goes wrong

A system this tightly coupled to the score needs an answer for the case where someone serves from the wrong box, and the Laws give an unusually forgiving one.

A service court error is made when a player has served or received out of turn, or has served or received from the wrong service court. If one is discovered, the error shall be corrected when the shuttle is not in play, and the existing score shall stand.

Reference The definition of a service court error and the instruction that it be corrected when the shuttle is not in play with the existing score standing, from Law 12.

The existing score stands. Rallies played from the wrong box are not replayed and points are not returned; the arrangement is simply put right and the game continues.

That is the same choice volleyball makes about a rotational fault discovered after a rally, and football makes about reviewing a goal after play has restarted. Three sports, three governing bodies, one instinct: once play has been built on a mistake, the cheapest correct answer is to fix the arrangement rather than unwind the game.

Why the serve is so heavily constrained

Everything above concerns where. The Laws are equally particular about how, and the reason is the shuttle.

A correct service requires the flight of the shuttle to be upwards from the server’s racket, passing over the net so that, if not intercepted, it would land in the receiver’s service court. Server and receiver must stand within diagonally opposite service courts without touching the boundary lines, and some part of both feet of both players must remain in contact with the surface in a stationary position from the start of the service until it is delivered.

Reference The requirements for a correct service — an upward flight from the racket, diagonally opposite service courts without touching the boundary lines, and both feet of both players stationary and in contact with the surface — from Law 9.1.

An upward flight, from a stationary stance, into a box whose shape depends on the format and whose side depends on the score. It is the most constrained action in the sport, and it has to be: a projectile this light and this heavily damped can be struck downward from a standing start with almost no warning, and a serve that could be hit down would end most rallies before they began.