On 29 July 1981, in front of St Paul's Cathedral in London, Diana and Charles kissed in public. It was not in the protocol. Until then the British court did not do things like that - a wedding was an act of state, not a love scene. What looked at the time like a small step out of line became a rule half of Europe still copies today.

Sarah Ferguson and Prince Andrew were the first to take it up, on 23 July 1986 at Westminster Abbey - and they did not stop at one kiss but at two. From there the copying spread across the continent.

Every court adapted it to its own taste

Mette-Marit and Crown Prince Haakon at Oslo cathedral on 25 August 2001. Maxima and Willem-Alexander in Amsterdam on 2 February 2002 - two kisses, the first short and shy, the second longer. Mary and Frederik in Copenhagen on 14 May 2004, and just eight days later, on 22 May, Letizia and Felipe at Madrid's La Almudena cathedral, with a kiss the sources describe as shy, somewhat solemn, but very affectionate.

Victoria and Daniel Westling in Stockholm on 19 June 2010. Then the most watched of them all: Kate and William on the balcony of Buckingham Palace on 29 April 2011, before more than half a million people gathered below - two kisses again. Charlene and Prince Albert in Monaco, at the start of July that same year, kept it short and discreet - Monegasque protocol does not yield easily.

Madeleine and Christopher O'Neill kissed on the steps of the palace chapel in 2013. Sofia and Carl Philip in 2015, where the prince held her face with both hands. Meghan and Harry on 19 May 2018 chose steps rather than a balcony - a kiss the press described as natural and full of love. Then Eugenie and Jack Brooksbank the same year, Gabriella Windsor in 2019, and Nina Flohr and Prince Philippos in Athens on 26 June 2021, amid white petals.

What actually happened

Forty-five years on, the kiss on the steps is a compulsory shot. The photographers wait for it, the public demands it, and any couple that does not perform it gets a headline for what they did not do. A gesture that arose as a departure from form became the new form - which is the fate of almost every tradition.

The irony is that the 1981 kiss is remembered today as the beginning of something warm, while the marriage it began ended the way it ended. Royal weddings have a way of producing a symbol that outlives the very thing it was meant to symbolise.