Question: The middle line of the front wall is relevant when?
About the article "Squash (sport)", section "The court".

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Basic rules and gameplay.
The court.
The squash court is a playing surface surrounded by four walls.
The court surface contains a front line separating the front and back of the court and a half court line, separating the left and right hand sides of the back portion of the court, creating three 'boxes' - the front half, the back left quarter and the back right quarter.
Both the back two boxes contain smaller service boxes.
All of the floor-markings on a squash court are only relevant during serves.
There are four walls to a squash court.
The front wall, on which three parallel lines are marked, has the largest playing surface, whilst the back wall, which typically contains the entrance to the court, has the smallest.
The out line runs along the top of the front wall, descending along the side walls to the back wall.
There are no other markings on the side or back walls.
Shots struck above or touching the out line, on any wall, are out.
The bottom line of the front wall marks the top of the 'tin', a half metre-high metal area which if struck means that the ball is out.
In this way the tin can be seen as analogous to the net in other racquet sports such as tennis.
The middle line of the front wall is the service line and is only relevant during serves.
American handball is played on a walled-court with either a single (front) wall, three walls, or in a fully enclosed four-wall court.
The four-wall court is a rectangular box.
The front wall is square, and the side walls are long and high.
In the middle of the floor of the court lies the short line, dividing the floor into two squares.
Also along the floor is the service line, which is in front of the short line.
The service zone is the area between these two lines.
The back wall of the court is usually high, with an above gallery for the referee and scorer, and also spectators.
A few courts have a glass back wall and/or glass side walls to allow for a better view of the match.
(In three-wall court handball, the court often has a front wall and two full side walls, or the front wall is flanked by two triangular wings.
)Handball may be played as singles (two players against each other), doubles (two teams of two players), or "cut-throat" (three players rotating one-against-two).
(In "cut-throat" handball, one server plays against two receivers, until he or she is put out.
Then, the left-most receiver serves, and so serves rotate in this way until one player scores 21 points and wins.)
The "cut-throat" mode of play is also known as "triangles".
The ball is "served" by one player standing in the service zone, by dropping the ball to the floor of the service zone and striking it on the bounce with the hand or fist so that it hits the front wall.
The ball must hit the front wall first; it may then hit at most one side wall; the served ball must pass the short line before the first bounce, but must bounce before reaching the back wall.
When the served ball lands in front of the short line, it is called a "short," while a serve which reaches the back wall without bouncing is called "long," and a serve which hits both side walls before bouncing is called a "3-wall".
All these are service faults.
If the server gets two faults in a row, he or she is out, and becomes the receiver.
If a serve hits the ceiling, floor, or a side wall before hitting the front wall, the server is out (no second serve allowed).
In doubles, the server's teammate has to stand in the service area with his/her back to a side wall in a service box, marked by a parallel line from the side wall, until the ball passes the short line.
The receiver must stand at least behind the short line, indicated by dashed lines extending from each side wall, while the server has the ball.
Once the ball is served, he or she must hit the ball either directly ("on the fly") or after the first bounce so that it bounces off the front wall.
However, if the receiver chooses to take the serve on the fly, he or she must first wait for the ball to cross the short line (the dashed line, in racketball).
The ball must not bounce off the floor twice.
Nor can any player during a return hit the ball off the floor before it touches the front wall.
The server then hits the ball on the rebound from the front wall, and play continues with the opponents alternatively hitting the ball until one of them fails to make a legal return.
After the serve and return, the ball may be played from anywhere, and may hit any number of walls and/or the ceiling, so long as it hits the front wall before bouncing on the floor.
Players cannot "hinder" (block) their opponents from hitting the ball.
If the server fails to make a legal return, he or she is out, and becomes the receiver.
If the receiver fails to make the return, a point goes to the server, who continues to serve until he or she is out.
So, only the server/serving team can score points.
The game goes to the player/team to score 21 points first, and a match goes to the player/team to win two out of three games; the third game goes to 11 points. 
Relevant may refer to: Relevant--- something when is on time  Relevant operator, a concept in physics, see renormalization group  Relevant, Ain, a commune of the Ain "département" in France  "Relevant Magazine", an Oldenburg horse famous in the sport of dressage  
A three-wall handball court is an outside court with a front-wall, two side-walls (these may be "full" or "halfs"—halfs being a pair of sloping side-walls), and no back-wall in the play area.
It is played very much like an indoor four-wall court, only with the challenge of returning the ball without any back-wall rebound.
The long line at the forty foot mark is considered "in" if the ball hits it when hitting the floor. 
There are many types of shots played that lead to interesting games and strategy.
Straight drive: The ball is hit parallel and close to a side wall to travel deep to the back of the court (the 'basic' squash shot).
Often referred to as a 'good length' shot.
Boast: The ball is played off a side wall at an angle, or the back wall, before hitting the front wall.
Volley: The ball is hit 'on the full' (before it touches the floor), usually directly to the front wall  Drop shot: The ball is hit gently against the front wall, to fall softly to the floor in the front corner.
Lob: The ball is hit softly and high on the front wall and with a high arc, so that it falls in a back corner of the court.
Cross Court: The ball is hit to the front wall from the right side to the left (or vice versa).
Kill: The ball is hit hard and low on the front wall so that it travels no farther than half court.
Trickle boast: A 'short' boast where the ball is hit to the side wall at the front of the court (often disguised as a drive or drop shot).
Squeeze boast: A more difficult shot which is hit from the front of the court when the ball is very close to the side wall.
Has the same effect as the trickle boast but is more deceptive because of its difficulty.
Nick shot: the ball is 'volleyed' or hit off a bounce to strike the front wall then the junction of the side wall and floor (the 'nick') giving the ball little or no bounce.
Rolling nick shot: When the nick shot is hit extremely well, the ball will roll along the floor (this is a more advanced shot that is a variation of the kill shot).
Back wall boast: the ball is hit moderately hard and high off the back wall, so that it goes the length of the room and hits (usually low) off the front wall.
Philadelphia (or corkscrew): A shot played diagonally upwards into the front corner hitting the front wall first and then the side wall.
The ball then lobs over the court with significant spin.
Ideally it hits the opposite side wall at the back and travels parallel to the rear wall making a return very difficult.
This shot is a favourite in exhibition squash but is susceptible to being volleyed.
Skid boast: A shot played from the back corners of the court where the ball is hit high along the sidewall with a small angle so that it hits the sidewall first, then hits high in the middle of the front wall continuing to cross the court while high in the air ideally hitting the opposite sidewall and landing close to the backwall to go past the opponent.
As with the Philadelphia it is suceptible to being volleyed.
Mizuki: This shot is hit as a volley.
Unlike a normal volley, the Mizuki shot is hit with the other side of the racquet by turning the wrist which deceives the direction of the ball.
Reverse angle: a boast where the opposite sidewall is hit first.