Black Rod knocks because the Commons has just shut its door in his face, and he is let in only after three knocks. The popular explanation is defiance. The door slam is usually read as the Commons showing its independence from the Crown, a gesture traced to the English Civil War.

The official explanation is plainer. Erskine May, the standard guide to parliamentary procedure, records that successive Speakers have ruled the custom lets the Commons establish Black Rod’s identity. It is not, they ruled, the direct assertion of the House’s rights that people often suppose.

Who Black Rod is

The full title is the Gentleman Usher of the Black Rod, a post created in 1350. At the State Opening, Black Rod’s job is to fetch MPs to the House of Lords to hear the monarch’s speech.

The ritual runs in a fixed order. The door shuts as Black Rod approaches, and Black Rod knocks on it three times. Once admitted, Black Rod tells the Speaker that the monarch commands the House to attend at once in the House of Peers.

The MPs then walk to the Lords in pairs. Aileen Walker, who worked at the House of Commons for over 30 years, describes their walk as more a casual saunter than a procession, with chatting and joking on the way.

A ceremony that means different things

The State Opening is the only regular occasion when the Commons, the Lords and the monarch meet together. The anthropologist Emma Crewe, who has studied both Houses from the inside, argues that each part of it says something different to each audience.

In her reading, the procession shows the monarch at the top and the peers just below. The Commons’ part, with the slammed door, is set up to challenge that order.

Some MPs added their own lines. The republican MP Dennis Skinner had a customary quip for Black Rod. In 1996 his quip was “New Labour, new Black Rod”.

The ceremony in its present form dates from 1852, after the Palace of Westminster was rebuilt following a major fire. Its roots go back to the 16th century. Crewe notes that for much of the 19th century the State Opening was scarcely an event at all.