There are only 427 seats in the Commons chamber to accommodate 646 MPs.
Hence on major parliamentary occasions, such as the Budget or prime minister's questions, there is always a scramble for the best seats.
Ordinary backbench members can only guarantee a seat for themselves by using a prayer card.
Behind each seat on the green benches is a small, brass frame into which MPs can place a card with their name.
This card must be put in place before prayers take place each day and the MP must be in that seat during prayers.
The seat is then reserved for that MP for the rest of the day.