A tramline in tennis refers to a part of the playing area of the court. On the sides of the rectangular tennis court, there are two lines that run parallel to each other. The inner line is called the singles sideline whereas the outer line is called the doubles sideline. Both these lines are called the tramlines. The area within these tramlines is called the alley. The alley on one side of the net is 39 feet long and 4.5 feet wide.
The significance of the tramlines is different in both singles and doubles tennis. In singles, if the ball lands in the alley, it is considered to be out of bounds. However, if even the smallest area of the ball touches the singles sideline, it is considered in play.
In doubles, the alley is a part of the playing area. Hence, the entire width of the court, till the doubles sideline, is considered within the playing area. In both singles and doubles, the service box area remains the same. A side of the service box is made by a part of the singles sideline. If a serve lands beyond this line, then the serve is considered as a fault. A line umpire usually calls where the ball pitches with respect to the sidelines.
The tramlines are usually absent in some of the courts that are specifically designed for singles events only. However, since most professional events also host a doubles tournament, the courts are designed with the alley as well.