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A.W. Purdue claims that until Princess Louise's marriage to the Marquis of Lorne in 1871, "no royal had married a subject since Henry VIII" ("Commoner chameleon", 25 November).

In fact, James II married Anne Hyde in 1659 (although the Stuart family was in exile at the time). In 1772, the Royal Marriages Act outlawed all marriages within the royal family that were not made with the sovereign's consent. Consequently, three marriages contracted by sons of George III to subjects were illegal.

It is a little-reported aspect of the royal engagement that the Queen formally gave her consent under this law on the morning of the announcement. Without it, Prince William and Kate Middleton's marriage would be illegal.

William Gibson, Professor of ecclesiastical history, Oxford Brookes University.