Image copyright Getty Images Image caption Choristers at King's College, Cambridge, rehearse their Christmas service

Everyone knows Silent Night, but contemporary carols are still being written. We meet two of the UK's most popular composers, John Rutter and Bob Chilcott.

"One of the nice things about Christmas music is that you get 11 months off from it."

So says "Mr Christmas" himself - better known as John Rutter, a choral tour de force, composer of some of the UK's most popular contemporary carols, and master of ceremonies at the Royal Philharmonic's annual Christmas concert.

But the 73-year-old, whose December is busier than Santa's, says he's never grown tired of the season.

"You do get 11 months off but by the time Christmas comes around, it's fresh again; and when you hear the same carols from year to year, it ties some of the threads of your life together.

"It's lovely, because we do look for reassurance in a changing world."

Rutter is one of the world's most successful, and most-performed, living composers; possessing a remarkable instinct for vocal melody, and a lightness of touch that makes his music accessible and enjoyable for performers and audiences alike.

Image caption John Rutter conducts a Christmas service at Dorchester Abbey in 2011

But it is as a writer of carols that he's really made his mark. He wrote his first, The Nativity Carol, when he was 16 years old and studying at London's Highgate College; and created the jolly, lyrical Shepherd's Pipe Carol not long after.

"I think the idea was my school choir would perform it at their festival of Nine Lessons and Carols," he recalls, "but I'm not sure they ever did - because at that point I left and moved on to University.

"I had no thought of it being published - but word reached the ears of Sir David Willcocks, who was very much associated with Christmas music and carols himself, and he asked to see it. Well, he actually asked to see all sorts of things - but the Shepherd's Pipe Carol might have been near the top of the pile; and he looked up at me and said, 'Would you be interested in this being published?' and that's how it all began."

Since then, Rutter has written more than two dozen carols, with the Donkey Carol, the Shepherd's Pipe Carol and The Very Best Time Of Year among the UK's most-performed contemporary choral works.

So how does he continue to find inspiration from the same Biblical story, year after year?

"That's like saying 'how do you continue to enjoy a beautifully-cooked meal?'" he laughs. "These are pleasures that stay with you and come around and around.

"People like that Christmas remains similar to how it was. When you hear a carol like Away in a Manger, it's like you're a child again. That's one of the things that makes Christmas so magical."

Rutter points out that carols are one of the oldest forms of English-language choral music - because the 15th century church allowed them to be sung in the common tongue, at a time when religious services were conducted in Latin.

The earliest mention in English of a "Christmas carol" appears in a 1426 work by John "The Blind" Audelay, a chaplain who made a list of 25 "caroles of Cristemas", sung by revellers who went from door to door during the festive season.

At this point, they were mainly folk songs, more frequently sung in pubs than churches; which is why so many of the lyrics don't quite work.

How, for example, did three ships come sailing into Bethlehem, a city that's a good 70km from the sea? The answer is they were probably camels, "the ships of the desert", but no-one's really sure.

And what have Holly and Ivy got to do with the nativity? They've been retrospectively claimed as a metaphor for Jesus's crown of thorns and the blood he shed, but that explanation is more apt for Easter than Christmas.

Media playback is unsupported on your device Media caption Carols

It wasn't until the Victorian era that carols became firmly associated with religious celebrations. That's all thanks to Edward White Benson, the first Bishop of Truro, who in 1880 devised a new type of service: The Nine Lessons And Carols.

Taking nine biblical passages about the Christmas story and interspersing them with rousing carols and hymns, his intention was (at least in part) to get his congregation out of the pub.

Despite the fact that Truro cathedral was still being built, meaning the first service had to be held in a temporary wooden shed, the Nine Lessons service was an immediate success.

Soon, it was being replicated in churches up and down the country; cementing its place on the Christmas calendar in 1918, when Cambridge's King's College took up the service. The BBC first broadcast the King's College concert 10 years later, and has done ever since.

Media playback is unsupported on your device Media caption John Rutter's Dormi Jesu, performed by King's College Choir

"It's true: Christmas tradition as we know it didn't start that long ago," says Bob Chilcott, once described as "a contemporary hero of British choral music" by the Observer, who has written and arranged a rich seam of Christmas music.

"We always had Christmas hymns but carols, in a sense, were just songs with a chorus that people could join in.

"You might say they're a form of folk art."

Chilcott has added to the canon with the gentle and reflective Shepherd's Carol and the charming Midnight Of Your Birth, written especially for children's choirs.

So is there any pressure to live up to the likes of Hark The Herald Angels or Silent Night when you're writing new carols?

"I find it rather rewarding," says Rutter. "You feel like you're adding one more little tile to a huge mosaic."

"It's nice to perpetuate the tradition with new carols," agrees Chilcott.

"We're quite good in this country. We're quite open to new ideas. I've found people like to sing the traditional ones, but they like to move on and try new things, too."

This includes Chilcott's hugely enjoyable arrangement of 12 Days of Christmas, in which every repetition of "five gold rings" is a diversion into a new genre - from jazz to gospel and even barbershop.

Always guaranteed a rapturous response, it was initially written for the 2002 Choir of the Year competition - but Chilcott reveals the musical pastiches were born of necessity.

"Five gold rings, just that little bit of the song, is still in copyright," he explains. "I cut that melody out and wrote my own little version to suit the style of each of the visiting soloists and groups.

"It's a bit of an eccentric arrangement but it works quite well."

Rutter, an old friend and mentor, is more effusive.

"It's so much fun!" he enthuses. "It's so much more than an arrangement of 12 Days of Christmas, it's an epic. It always brings the house down."

12 Days will close the first half of the show this weekend when the two composers share a podium for the first time at the Barbican in London, accompanied by the BBC Singers and the BBC Concert Orchestra.

For the audience, the concert will provide more than enough Christmas music to last 'til 2019 - but Rutter and Chilcott don't have it so easy.

"For many choirs, Christmas is their main concert of the year, so they start preparing and casting out for ideas in the spring," says Chilcott. "So I often write a lot of Christmas music straight after Christmas itself.

"It's a good time to do it because it gets a bit strange if you start writing about the snow in the middle of the summer".

Christmas Carols with Bob Chilcott and John Rutter takes place on Saturday 15 December at the Barbican in London. The concert will be recorded for future broadcast on BBC Radio 3 and BBC Sounds.

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