What are royal Christmas cards trying to tell us?

It's become a seasonal tradition to seek the hidden message or symbolic meaning in the Christmas cards the royals send out, as they keep changing and reinventing the format.

This year's card from King Charles and Queen Camilla shows them looking relaxed and maybe relieved - and there is a very personal significance behind this picture.

It was the first photoshoot after the King was given the green light that he was well enough to return to public duties, after beginning his cancer treatment. It was said to be a watershed moment for the couple, caught on camera.

The same pictures, with images full of spring rebirth, were then used for the official announcement that the King had made sufficient progress with his treatment to go back to public events.

The lack of snowy steeples and robins isn't a surprise, as royal Christmas cards rarely seem to show any signs of winter.

And the message, printed in red, always looks like a party invitation from the 1950s.

Prince Harry and Meghan have given their own twist to royal cards. They've added some glitz, so that it has the feel of film credits as much as a season's greeting.

It's an upbeat Californian message, sent out as an e-card, with six pictures rather than a single image, showing the couple hugging and laughing. It also drew comments on the rare appearance of their son and daughter.

If cards could have an accent, this would undoubtedly sound American. It's a "Happy Holiday Season", with no mention of "Christmas". But then, they've spent most of their married life in the US.

Prince William and Catherine's cards have used more informal pictures in recent years. It's jeans and no ties, a modern family, without any royal imagery.

This year's card kept the same relaxed style, but it had a very poignant significance. It was from the video that announced that Catherine had completed her chemotherapy.

It shows William and Catherine and their three children in Norfolk in August, from a video that was full of end-of-summer colours and very emotional messages about a tough year since her cancer diagnosis.

It was a strikingly different style of royal communication, unashamedly about love and togetherness - and they've used it again for the Christmas card.

Last year's card from the Prince and Princess of Wales had also been a talking point. It featured the same jackets-off, casual image, but there was also a designer chic, with an arty black-and-white picture that wouldn't have looked out of place in an upmarket jeans advert.

The prince is very keen on sustainability, so maybe next year's will be made out of recyclable seaweed.

Christmas cards can also be like time capsules, holding a moment.

In 1995 Prince William appeared alongside his mother Diana, Princess of Wales, and his brother Prince Harry in this rather haunting image. It really evokes another era.

There's often a sense of family closeness projected by the cards.

The late Queen Elizabeth II was always pictured with Prince Philip. And King Charles and Queen Camilla have continued to use images of themselves as a couple.

That's had to be mixed up with some props over the years.

For the 2019 card the then Prince Charles and Camilla were pictured in a vintage sports car, in a photo taken on a trip to Cuba. It was more or less made for a Prince of Wheels headline.

There was also a picture of the Royal Family standing around a speed boat in 1969, looking like winners on a game show.

Christmas cards might be slipping out of fashion - sales of boxes of cards are down 23% in a year, according to retailers John Lewis.

But the royals show no sign of losing interest - and that includes European royal families... although their use of a family group in a posh room isn't always that original.

The Belgian royal card has a multi-lingual message, which is inclusive and reflects a multi-lingual country, but risks looking like a Eurostar menu. It's also unusually forward-looking, with the date of 2025.

Spanish royals this year used their card to send a more serious message. There was a standard family group photo on the front, but inside was a poem that was a tribute to the victims of the Valencia flood.

Last month, Spain's king and queen had been pelted with mud when they visited areas hit by the floods.

You couldn't say that the Christmas card pictures are always predictable or easy to interpret.

What was the thinking behind the 2016 card which used a photo of Prince Charles and Camilla on a trip to Croatia? An unexpected Eurovision entry?

They might begin as greetings cards, but they soon become history. Like this poignant wartime Christmas card from the then Princess Elizabeth, sent in 1942. There's the tilt of the cap, the young face, looking into an unknown future.

There's often a hint of melancholy in Christmas films and songs, hinting at the passing of time, and that's here, too.

Happy Christmas! It's in the post.