Line-out code
In a line-out with both teams separated by a gap of a metre and the ball being thrown down the middle there should be an equal chance for each team to retreive the ball. To give their own team an advantage they will make a line-out call. The call will be made by a previously selected player - usually the scrum-half, though it may be the pack-leader or even the hooker.
Before the game starts the team will agree on a code. One method is to specify FRONT, MIDDLE or BACK of the lineout by using a three word sentence with unique letters. For example "THE"(front) "BIG"(middle) "DON"(back). Prior to the throw the hooker will shout a three letter code, for example "D" "Z" "E". The team will have agreed on which letter out of the code to use and they will throw away the other two, they are there only to make decryption of the code harder. If the chosen letter to use for all line-outs was the first one, then the only word that has "D" in it is "DON". Therefore the team-mates know the ball is going to be thrown to the back of the lineout.
In addition to specifiying how far the ball will be thrown the code may also include instructions for what to do with it - catch and hold to form a maul, tap it back to the scrum half, catch it and the pass it to another line-out player who has peeled off the line and is charging round to either the front or back of the line.
So a call of "H Z F Red Orange" May be an instruction for the ball to be thrown to the front of the line then caught and passed to the player from the rear of the line who has left his position and is charging around to the front of the line, and then for him to pass the ball back to the hooker who waits where he was on the line and can now run infield.
Moves and calls get very complex and over-complexity can often lead to the move breaking down. Teams will often spend a lot of time working on moves and calls, and ensuring all players understand their roles.