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.