She died at 75, on 2 September, and the news only became public now - ten days later, when the family decided it was time. Davina Sheffield was not a queen, was not a princess and never entered any official Buckingham Palace protocol. And yet in the seventies the British press considered her the woman who nearly took a seat on the throne beside Charles.

The relationship began in 1976. Charles, then Prince of Wales, took her to social occasions, to a wedding, and at one equestrian event she sat in a place reserved for the closest family - beside Queen Elizabeth II and the Duke of Edinburgh. She travelled with him to Balmoral too, where she won over the Queen Mother as well. Royal commentators at the time wrote that he had found his "soulmate".

Then came the sentence that turned all of it around. Her former partner, the sailor James Beard, said publicly that the two of them had been in a relationship and had lived together before she met the prince. That was enough. The tradition of the time required the future king to choose a bride with an "impeccable" past, and Davina Sheffield suddenly stopped meeting the condition - not because of anything she had done, but because of what somebody else said about her.

It is worth pausing here. A woman descended from the baronial Sheffield family, whose mother was the daughter of the first Baron McGowan and chairman of Imperial Chemical Industries, whose father served in the Grenadier Guards - she, then, was not good enough because she had a life of her own before the prince. How much of that rule has survived to this day, and how much of it has simply moved from the newspapers to the networks?

Historians later uncovered a detail that sounds invented. Davina's ancestors, on the Sheffield line, owned Buckingham Palace itself - back when it was still called Buckingham House, hundreds of years before it passed into the hands of Queen Charlotte. The woman not let into the palace descended from the family that built it.

After parting from Charles she married Jake Morley in 1981 - the same year Charles wed Diana. They had a son, Thomas, in 1983, then William and Henry in 1987. The same names borne by Charles's two sons. A coincidence, of course, but the kind you do not invent. All three were with her to the end; she will be laid to rest on 2 November, at a funeral service near her home in the Cotswolds.

After her, Charles was linked with Jane Wellesley, Amanda Knatchbull, Sabrina Guinness, Sarah Spencer and finally with Diana Spencer. We know how that ended: divorce in 1996, tragedy in Paris a year later. And the man who in 1976 was not permitted to choose a woman with a past married Camilla in 2005 - a woman he had been involved with back in the seventies, and who had in the meantime been married to Andrew Parker Bowles.

In other words, the rule that took Davina Sheffield out of the running was later simply discarded. Not corrected, not publicly re-examined - just quietly set aside when it inconvenienced the very man in whose name it had been applied. That is the difference between a principle and a tool, and that difference is rarely admitted while everyone is still alive.