A minimum number of politicians must be present when the Houses are sitting to make Parliamentary decisions valid.
This minimum number is known as a quorum.
In the Commons the quorum is 40 Members of Parliament, including the Speaker.
There is no need for a quorum to be present at all times - in fact, Commons debates could theoretically continue even if attendance in the chamber dwindled to just one MP and the Speaker.
But if a division is called and fewer than 40 MPs are present to vote, then a decision on the business being considered is postponed and the House moves on to consider the next item of business.
The quorum in the House of Lords for votes on legislation is 30, but a minimum of three peers, including the Lord Speaker, need to be present for a debate to take place.