Members of the Government occupy the benches on the Speaker's right, whilst members of the Opposition occupy the benches on the Speaker's left. In front of each set of benches a red line is drawn, which members are traditionally not allowed to cross during debates. The Prime Minister and the government ministers, as well as the leader of the Opposition and the Shadow Cabinet sit on the front rows, and are known as "frontbenchers". Other members of parliament, in contrast, are known as "backbenchers". Not all Members of Parliament can fit into the chamber at the same time, as it only has space to seat approximately two thirds of the Members. According to Robert Rogers, former Clerk of the House of Commons and Chief Executive, a figure of 427 seats is an average or a finger-in-the-wind estimate. Members who arrive late must stand near the entrance of the house if they wish to listen to debates. Sittings in the chamber are held each day from Monday to Thursday, and also on some Fridays. During times of national emergency, the house may also sit at weekends.