Visitors are often surprised that the MPs' chamber seems much smaller than they had expected.

The rows of green benches (b) accommodate only 437 of the 659 MPs, which is why many have to crowd at either end or sit on the carpeted steps during major debates and statements.

Traditionally, frontbench speakers should never step over the red lines on the floor - the two lines are exactly two sword-lengths apart (a) - as, theoretically, the MPs would be able to attack one another.

The prime minister and leader of the opposition speak from the 'despatch boxes' (c) in the centre of the chamber, overlooked by the Speaker and backbenchers.

Visitors can stand where the prime minister stands at Question Time to see the view he sees (e), or get the effect of being a backbencher from farther back. But no visitor is ever allowed to take a seat.

The Speaker's chair - with a lighted canopy - was originally a present from Australia and is the only wood of its type in the Houses of Parliament (d). It is made of Australian Black Bean Wood - everything else is oak.

MPs who have collected names on a petition can put it in a specially-made sack on the back of the Speaker's chair, ready for it to be considered by the government.