The Monday ruling by John Bercow, speaker of the House of Commons, against allowing a third vote on Prime Minister Theresa May's Brexit deal in its current form consumed the news cycle in Britain. (Daniel Sorabji/AFP/Getty Images)

Britain’s 2016 vote to leave the European Union was supposed to be about imagining a new, independent future for the nation. But as Brexit moves forward, it has often ended up mired in the past, with centuries-old disputes — including over the Irish border or the status of Gibraltar — proving to be newly intractable.

As such, it seems only fitting that the latest blow to Brexit involves a 1604 legal precedent. This is what John Bercow, speaker of the House of Commons, cited Monday when he announced he would not allow Prime Minister Theresa May’s government to hold a third vote on her withdrawal agreement in its current form.

In his ruling, about which the prime minister’s office has said it was not forewarned, Bercow quoted from a guide to parliamentary procedure that said a question that “has been decided during a session may not be brought forward again during that same session.” He said this is a “strong and long-standing convention” dating to 1604.

It is a turn of events that has shocked many in Europe. “I’ll concede that I wasn’t actively aware of the British Parliament’s rules of procedure from the 17th century, so I took note of this with interest yesterday,” German Chancellor Angela Merkel said Tuesday.

But why is Bercow harking to a legal idea from the era of Shakespeare (fun fact: 1604 featured the first known performance of “Othello”) in a decision that could lead Britain to a potentially disastrous “no deal” Brexit? Here’s a rough guide.

What was the guide to parliamentary procedure from which Bercow was quoting?

Bercow’s decision was based on an official parliamentary rule book that was first published by Thomas Erskine May in 1844. This book, known as “Erskine May,” has become an important tool for understanding Britain’s constitution, which, unlike the U.S. Constitution, was not codified in a single document and is derived from a set of laws and conventions.

“Erskine May” has been regularly updated, with 24 editions so far. A new edition is expected in May. In print form, the book is 1,097 pages and will cost you £439.99 ($583.20) in Parliament’s bookshop.

In his statement, Bercow said he was referring to a statement on Page 397 of the 24th edition of the book: “A motion or an amendment which is the same, in substance, as a question which has been decided during a session may not be brought forward again during that same session.”

The book goes on to state: “Attempts have been made to evade this rule by raising again, with verbal alterations, the essential portions of motions which have been negatived. Whether the second motion is substantially the same as the first is finally a matter for the judgment of the Chair.”

Britain’s current parliamentary session began in 2017, with May’s government approving an unusual two-year session, rather than a one-year session, in a bid to “deliver Brexit.” The withdrawal agreement May reached with E.U. leaders has been voted on twice in some form, in January and earlier this month. It has been rejected both times.

Britain’s Supreme Court decided in January 2017 that Parliament must have a “meaningful vote” on the terms of leaving the E.U., so parliamentary approval of May’s deal is vital.

So, what is this about 1604?

In his statement, Bercow suggested there had been past instances of the speaker of the House of Commons applying this rule, “notably in 1864, 1870, 1882, 1891 and 1912.” However, he said the initial precedent dated to April 2, 1604, when it was put into practice by Parliament during the speakership of Sir Edward Phelips.

The context for this rule is interesting. This was a complicated time in British political history — King James VI of Scotland had become James I of England and Ireland with the union of the Scottish and English crowns the year earlier. Josh Chafetz, a professor at Cornell Law School who has written about this moment in legal history, says the English Parliament was suspicious of the newly powerful Scottish royal and sought to codify its powers.

Phelips was viewed as close to the monarchy, Chafetz wrote on Twitter, so the 1604 decision “both forestalls the sort of dilatory tactics that keep the House from turning to other business and also makes it harder for the Crown to keep bullying members until they vote the way that it wants.”

The next year, incidentally, a group of dissident Catholics would try to blow up Parliament and kill James. The infamous Gunpowder Plot failed, and the perpetrators who survived, including Guy Fawkes, were sentenced to be hanged, drawn and quartered. Phelips was involved in the legal case against the plotters.

Is Bercow’s decision legally sound?

That is a big subject of debate right now. Critics of Bercow’s decision might argue the precedent has not been invoked by a speaker since 1920 and that it is evidence of Bercow’s too-proactive use of his position and anti-Brexit views.

Others might say, however, that it is very much the speaker’s job to ensure parliamentary procedures are adhered to and that Bercow’s use of the precedent is a necessary pushback against May’s repeated attempts to pass her deal virtually unchanged.

May’s government may ultimately be able to circumvent Bercow’s decision, even if it is legally sound. The government may be able to end the current parliamentary session, for example, rework the motion proposed to Parliament, or simply ask the House of Commons to suspend the standing order that prevents repeat votes.

But there is a big problem: With the deadline for Britain’s exit from the E.U. 10 days away, there is not a lot of time left.

Read more:

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How Gibraltar almost stopped a Brexit deal