What is a let (law 2.9) in Table Tennis?

Section 5: Rules of Play

5.1 A Good Service (Law 2.6)

Per Law 2.6, service shall proceed as follows:

Service shall start with the ball resting freely on the open palm of the server's stationary free hand.
The server shall then project the ball near vertically upward, without imparting spin, so that it rises at least 16 cm (6.3 in) after leaving the palm, and then falls without touching anything before being struck.
As the ball is falling, the server shall strike it so that it touches first the server's court and then, after passing over or around the net assembly, touches directly the receiver's court; in doubles, the ball shall touch successively the server's right half-court and then the receiver's right half-court.
From the start of service until the ball is struck, it shall be above the level of the playing surface and behind the server's end line, and it shall not be hidden from the receiver by the server or the server's doubles partner or by anything they wear or carry.
As soon as the ball has been projected, the server's free arm and hand shall be removed from the space between the ball and the net.
It is the responsibility of the player to serve so that the umpire or assistant umpire can be satisfied that the service conforms with the Laws. If either umpire is not sure of the legality of a service, they may warn the server on the first occasion in a match; any subsequent such service shall result in a point to the receiver.
Whenever there is a genuine inability to comply (e.g., due to physical disability), the umpire may relax requirements relating to the free hand.

5.2 A Good Return (Law 2.7)

Per Law 2.7, the ball, having been served or returned, shall be struck so that it passes over or around the net assembly and touches the opponent's court, either directly or after touching the net assembly.

5.3 The Order of Play (Law 2.8)

Per Law 2.8:

In singles, the server shall first make a good service, the receiver shall then make a good return, and thereafter server and receiver shall alternately make a good return.
In doubles, the server shall first make a good service, the receiver shall then make a good return, the server's partner shall then make a good return, the receiver's partner shall then make a good return, and thereafter each player shall make a good return in sequence.

5.4 A Let (Law 2.9)

Per Law 2.9, a rally is a let if:

In service, the ball touches the net assembly, provided the service is otherwise good or the ball is obstructed by the receiver or the receiver's partner.
Service is delivered when the receiving player or pair is not ready, provided neither the receiver nor the receiver's partner attempts to strike the ball.
A player fails to make a good service or a good return, or otherwise infringes the Laws, as a result of a disturbance outside the player's control.
Play is interrupted by the umpire or assistant umpire.
The rally is interrupted to correct an error in the order of serving, receiving, or ends.
The rally is interrupted to introduce the Expedite System.
A let is replayed without any change to the score.

5.5 A Point (Law 2.10)

Per Law 2.10, a player scores a point unless a let is called, if the opponent:

Fails to make a good service.
Fails to make a good return.
Strikes the ball twice successively (double hit).
Strikes the ball with a side of the racket blade whose surface does not comply with Law 2.4.
Moves the playing surface or net assembly while the ball is in play (Law 2.10.1.7 and 2.10.1.8).
Touches the playing surface with the free hand while the ball is in play.
In doubles, strikes the ball out of the proper sequence.
Is wearing, carries, or touches the ball while wearing or using a racket that does not conform.
As a receiver under the Expedite System, the receiver's side completes 13 returns in a rally (the server scores one point).

5.6 A Game (Law 2.11)

Per Law 2.11, a game is won by the player or pair first scoring 11 points, unless both players or pairs score 10 points (deuce), when the game shall be won by the first player or pair subsequently gaining a lead of 2 points.

5.7 A Match (Law 2.12)

Per Law 2.12, a match shall consist of the best of any odd number of games. At top-level competition, matches are typically best-of-7 games for World Championships and Olympic Games, and best-of-5 or best-of-7 for WTT events. National competitions may use best-of-3 or best-of-5.

5.8 The Order of Serving, Receiving, and Ends (Law 2.13)

Per Law 2.13:

The right to choose initial order of serving, receiving, or ends shall be decided by lot (coin toss or similar); the winner may choose to serve or to receive first, or to start at a particular end.
Service shall change after every 2 points until the end of the game or the score reaches 10-all, after which service shall change after every point.
In the deciding game of a doubles match, the pair that is due to serve shall change its order of service when either pair first scores 5 points.
The player or pair serving first in a game shall receive first in the next game of the match.
In the last possible game of a doubles match, the receiving pair shall change its order of receiving when either pair first scores 5 points.
The player or pair starting at one end in a game shall start at the other end in the next game, and in the last possible game players or pairs shall change ends when either player or pair first scores 5 points.

5.9 Out of Order of Serving, Receiving, or Ends

Per Law 2.14, if a player serves or receives out of turn, or fails to change ends when required, play shall be interrupted by the umpire as soon as the error is discovered. Play shall resume with the service being made by the player whose turn it is to serve according to the sequence established at the beginning of the match. In all circumstances, all points scored before the error was discovered shall be counted.

5.10 The Expedite System (Law 2.15)

Per Law 2.15:

The Expedite System shall come into operation if a game is unfinished after 10 minutes of play, or at any earlier time at the request of both players or pairs.
If the ball is in play when the time limit is reached, play shall be interrupted by the umpire and the rally is replayed as a let.
Thereafter, each player shall serve for 1 point in turn.
Under the Expedite System, if the serving player or pair makes 13 returns in a rally, the receiver scores a point.
Introduction of the Expedite System does not alter the order of serving and receiving.
Once introduced, the Expedite System remains in operation for the remainder of the match.

Section 5: Rules of Play

5.1 A Good Service (Law 2.6)

Per Law 2.6, service shall proceed as follows:

Service shall start with the ball resting freely on the open palm of the server's stationary free hand.
The server shall then project the ball near vertically upward, without imparting spin, so that it rises at least 16 cm (6.3 in) after leaving the palm, and then falls without touching anything before being struck.
As the ball is falling, the server shall strike it so that it touches first the server's court and then, after passing over or around the net assembly, touches directly the receiver's court; in doubles, the ball shall touch successively the server's right half-court and then the receiver's right half-court.
From the start of service until the ball is struck, it shall be above the level of the playing surface and behind the server's end line, and it shall not be hidden from the receiver by the server or the server's doubles partner or by anything they wear or carry.
As soon as the ball has been projected, the server's free arm and hand shall be removed from the space between the ball and the net.
It is the responsibility of the player to serve so that the umpire or assistant umpire can be satisfied that the service conforms with the Laws. If either umpire is not sure of the legality of a service, they may warn the server on the first occasion in a match; any subsequent such service shall result in a point to the receiver.
Whenever there is a genuine inability to comply (e.g., due to physical disability), the umpire may relax requirements relating to the free hand.

5.2 A Good Return (Law 2.7)

5.3 The Order of Play (Law 2.8)

Per Law 2.8:

In singles, the server shall first make a good service, the receiver shall then make a good return, and thereafter server and receiver shall alternately make a good return.
In doubles, the server shall first make a good service, the receiver shall then make a good return, the server's partner shall then make a good return, the receiver's partner shall then make a good return, and thereafter each player shall make a good return in sequence.

5.4 A Let (Law 2.9)

Per Law 2.9, a rally is a let if:

In service, the ball touches the net assembly, provided the service is otherwise good or the ball is obstructed by the receiver or the receiver's partner.
Service is delivered when the receiving player or pair is not ready, provided neither the receiver nor the receiver's partner attempts to strike the ball.
A player fails to make a good service or a good return, or otherwise infringes the Laws, as a result of a disturbance outside the player's control.
Play is interrupted by the umpire or assistant umpire.
The rally is interrupted to correct an error in the order of serving, receiving, or ends.
The rally is interrupted to introduce the Expedite System.
A let is replayed without any change to the score.

5.5 A Point (Law 2.10)

Per Law 2.10, a player scores a point unless a let is called, if the opponent:

Fails to make a good service.
Fails to make a good return.
Strikes the ball twice successively (double hit).
Strikes the ball with a side of the racket blade whose surface does not comply with Law 2.4.
Moves the playing surface or net assembly while the ball is in play (Law 2.10.1.7 and 2.10.1.8).
Touches the playing surface with the free hand while the ball is in play.
In doubles, strikes the ball out of the proper sequence.
Is wearing, carries, or touches the ball while wearing or using a racket that does not conform.
As a receiver under the Expedite System, the receiver's side completes 13 returns in a rally (the server scores one point).

5.6 A Game (Law 2.11)

5.7 A Match (Law 2.12)

5.8 The Order of Serving, Receiving, and Ends (Law 2.13)

Per Law 2.13:

The right to choose initial order of serving, receiving, or ends shall be decided by lot (coin toss or similar); the winner may choose to serve or to receive first, or to start at a particular end.
Service shall change after every 2 points until the end of the game or the score reaches 10-all, after which service shall change after every point.
In the deciding game of a doubles match, the pair that is due to serve shall change its order of service when either pair first scores 5 points.
The player or pair serving first in a game shall receive first in the next game of the match.
In the last possible game of a doubles match, the receiving pair shall change its order of receiving when either pair first scores 5 points.
The player or pair starting at one end in a game shall start at the other end in the next game, and in the last possible game players or pairs shall change ends when either player or pair first scores 5 points.

5.9 Out of Order of Serving, Receiving, or Ends

5.10 The Expedite System (Law 2.15)

Per Law 2.15:

The Expedite System shall come into operation if a game is unfinished after 10 minutes of play, or at any earlier time at the request of both players or pairs.
If the ball is in play when the time limit is reached, play shall be interrupted by the umpire and the rally is replayed as a let.
Thereafter, each player shall serve for 1 point in turn.
Under the Expedite System, if the serving player or pair makes 13 returns in a rally, the receiver scores a point.
Introduction of the Expedite System does not alter the order of serving and receiving.
Once introduced, the Expedite System remains in operation for the remainder of the match.

5.4 A Let (Law 2.9)