It hasn't been used by a Commons Speaker for almost 100 years.
So when the modern day incumbent John Bercow referenced a parliamentary convention dating back to 1604 that could hamper Theresa May's hopes of third meaningful vote, there was understandable consternation in Westminster.
The 400-year-old principle dictates that MPs cannot vote on an identical matter twice in the same parliamentary session. The implication of Mr Bercow's remarks is that a third meaningful vote on Brexit cannot be held unless there is a "substantial change" to the deal.
Quoting from ‘Erskine May: Parliamentary Practice’ Mr Bercow said that one of the reasons the rule has lasted so long was that it was "necessary to ensure the sensible use of the House’s time and the proper respect for the decisions it takes".
‘Erskine May’ is an official parliamentary rulebook first written by constitutional expert Thomas Erskine May, who in 1844 published a work on the proceedings of parliament.
Now in its 24th edition, the guide has grown from 496 pages in 1844 to over 1,000 pages today. The latest edition, due to be published in May, will be the first version of the document available free of charge online.
The crucial ruling by the House Speaker comes from page 397 of the document, which states that a motion or amendment "which is the same, in substance" as the motion already rejected should not be brought forward again in a session of parliament.
As stated by Mr Bercow, who himself voted Remain, the move was in response to the questions from other MPs - one of them Labour’s Chris Bryant. It was Mr Bryant who gave a series of examples where speakers had refused MPs’ permission to raise issues, which had already been voted down in that session. They included cases from more than 300 years ago.
By rule, Speaker Bercow's interpretations of House of Commons conventions are binding, unless there is a majority vote to break with an invoked ruling.
In order to avoid a further stalling of the withdrawal agreement, the British government may now have to come back with essentially the same deal but tweaked with additional side agreements with the EU.
Prime Minister Theresa May could also introduce some changes to the non-binding Political Declaration on the future relationship between UK and the EU.
As expected, Speaker Bercow said he would "have to look at the particulars" of any such changes to decide whether they are "in order".
Alluding to the suggestion that the ruling could be by-passed by obtaining another opinion from Attorney General Geoffrey Cox, Mr Bercow said that "simply a change in an opinion about something wouldn’t itself constitute a change in the offer".
