Prince Charles stepped in for ailing Queen Elizabeth II on Tuesday to open Britain’s Parliament, a deeply symbolic moment in British royal history that gave the clearest glimpse yet of the contours of the post-Elizabethan world.

Her Majesty left her decision to Monday afternoon to finally pull the plug on her scheduled role of opening the new legislative session, inevitably sparking fresh concerns that Her Majesty is considerably more unwell than the palace—which cited “episodic mobility problems”—is letting on.

Notably, Prince Charles, who was decked out in his military uniform, sat on a throne to read the Queen’s Speech (which is actually written by the government but traditionally delivered by the monarch) although it had previously been rumored he would avail of more humble seating options. Her Majesty’s crown rested symbolically on a velvet cushion on a small table next to him. Prince William sat on Charles’ right, wearing a dark morning suit; Camilla, his wife, sat on Charles’ left. They both sat on relatively normal chairs.

While the queen would traditionally refer to “my government,” Charles referred to “Her Majesty’s government” throughout the speech.

The palace said William and Charles were jointly opening Parliament, in their roles as counselors of state, and that the queen was watching the ceremony on television at Windsor Castle.

The decision of Elizabeth to pull out of the event at the last minute has served to deepen national anxiety about her health, which has become a source of much comment since the death of her husband, Prince Philip, last year.

The official line is that the queen has “mobility problems”—the terminology was updated in a briefing Monday to become “episodic mobility problems”—but this explanation is increasingly thought to be a major understatement, especially in light of the legal mechanism that the queen used to step away from Tuesday’s commitment: the Regency Act.

The only time Britain has had a formal regency is under George III, who was periodically declared insane. George IV, the heir, was made regent and took on all his father’s duties.

It is not unprecedented for the monarch not to make the ceremony. The queen has actually missed the state opening of Parliament twice, while she was pregnant, in 1959 (with Anne) and 1963 (with Edward). Queen Victoria regularly skipped this particularly duty, sending the crown on a velvet cushion, just as the queen did today.

But an important if slightly obscure point is that on those occasions the monarchs asked a senior parliamentary official to fill in for her, essentially, an ad hoc basis.

The big difference concerning Monday’s move was that the queen a) put her heir, Charles, in her place rather than a mere functionary and b) according to a report in the Daily Mail by royal historian Robert Hardman, she did so by invoking the Regency Act of 1937 and, specifically, Section 6 (1) which states: “In the event of illness… the Sovereign may… delegate, for the period of that illness or absence… such of the royal functions as may be specified.”

This could be interpreted to mean that Charles is now authorized to open Parliament—really the monarch’s key constitutional function—for as long as the queen continues to say she doesn’t feel up to it for health reasons.

It’s certainly hard to imagine the queen now ever reprising the role.

It is hardly surprising that the invoking of the Regency Act on as important occasion as the opening of Parliament is being seen by many as the first step toward a regency, although this is something Buckingham Palace calmly but robustly denies, insisting the queen still remains very much in charge.

In most senses, she is. There is little doubt that her word is the final one.

But today’s appearance by Prince Charles in opening Parliament, supported by Prince William, marks an important line in the sand, and a window into what the future of British pomp and circumstance looks like.