A mace is a staff of office symbolising authority. The mace is carried in and out of the Commons and Lords Chambers in a procession at the beginning and end of each day. Without the mace in position, each House cannot sit and debate.
In the House of Commons the mace lies on the table in front of the Speaker when MPs are debating. If there is a Committee of the Whole House then the mace is placed under the table.
The mace is carried in and out of the Chamber by the Serjeant at Arms in a procession at the beginning and end of each day.
The House of Lords has two maces dating from the reigns of Charles II and William III. One of the maces accompanies the Lord Speaker into the Chamber where the mace is placed on the Woolsack behind the Lord Speaker when the House is sitting (except at the State Opening of Parliament when the Sovereign is present).
Further information on this subject can be found from the following links.
Photograph of the House of Commons Mace
Related glossary term: Serjeant at Arms
Related glossary term: Woolsack