Frequency: Once a year, with four gaps in the sequence: 1936 (too soon after the abdication crisis); 1938 (George VI didn't feel like it); 1959 (the Queen was seven months pregnant); and 1969 (too soon after the film Royal Family).
Audience: 13.4 million watched last year on BBC1. No figures available for ITV or BBC2, but an educated guess would put the total at 20-25m - plus many more around the Commonwealth.
Conception: 30 Oct 1923. John Reith, General Manager of the BBC, to Lord Stamfordham, the King's Private Secretary: 'It is our earnest desire that His Majesty the King should honour us by broadcasting a special message on Christmas or New Year's Day . . .'
Palace reaction: Negative.
Reith's response: He kept trying. The breakthrough came in July 1932, after George V visited the BBC's new HQ at Portland Place.
How was it for the King? 'At 3.35 I broadcasted a short message of 251 words to the whole Empire,' he wrote in his diary on Christmas Day 1932.
So the broadcast was live? Yes. The nation gathered round its wireless at 3.05pm.
That's half an hour early: No it's not. Until 1952, clocks at Sandringham were set 30 minutes fast to maximise daylight hours for shooting pheasant.
But no mishaps? Unfortunately, yes. Just before the broadcast, the King fell through his wicker armchair. Like a true pro, he exclaimed 'God bless my soul]' and delivered his lines (written by Rudyard Kipling).
Any surprises? His subjects were intrigued to discover that the King spoke with a slight German accent, a relic of his Saxe-Coburg ancestry.
Was it a hit? 'The greatest broadcast ever' was one newspaper's verdict.
A hard act to follow: Especially if, like George VI, you had a serious stammer.
How did he cope? He had a speech therapist who coached him before each broadcast. Until 1951 the system worked, and the message went out live.
What happened in 1951? In October, the King had an operation for cancer and lost a lung. He could scarcely breathe, so the message was pre-recorded in gobbets. The tape was then stitched into a seamless broadcast. Six weeks later, the King was dead.
Which brings us to the Queen: Indeed. She has already delivered twice as many Christmas broadcasts as her father and grandfather put together.
Another professional, then? Not to begin with. At the end of her first broadcast, viewers saw her smiling with relief at Prince Philip. Over the years, she has learnt the tricks of the trade. 'Red and green should never be seen,' she remarked to Philip Gilbert before the 1991 recording, realising she had chosen the wrong dress for the background.
Gilbert is the director? Yes, he took over from David Attenborough in 1991.
And who writes the script? She does - with a little help from her Private Secretary. What's more, because she delivers the message as head of the Commonwealth, there is no vetting by Downing Street.
So it's controversial stuff? We're talking about the Queen. In a typical year - and all years are typical - she talks about family, service and duty.
The annus horribilis wasn't a typical year? No, a tape of the broadcast found its way to the Sun, which published the transcript two days before Christmas. The Queen sued for breach of copyright, and the two parties settled out of court. But the broadcast itself was typical. A fleeting mention of her family's 'difficult' year, and then a restatement of the usual themes.
The bottom line - is it good? Yes, and getting better. Its secret is that it doesn't change - the more the Queen repeats herself, the more it seems to belong to a timeless tradition. And unlike most Christmas programmes, it's utterly sincere. Richard Tomlinson
York on Ads and Curtis on Video return on 2 Jan.
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