To be at the top of the charts on Christmas Day is to have a place in history.

Sometimes, serious pop acts have managed it:

the Spice Girls did it three times in a row.

Sometimes, it was more of a mid-90s fever dream.

♪ “Blobby, oh Mr Blobby…” ♪

Some of the importance of the Christmas Number One

comes from the UK music industry's official charts,

which are based purely on music sales, not on airplay.

Every pop music radio station in Britain could play a track every hour, on the hour…

and it wouldn't make a bit of difference if no one walked into a music shop,

opened their wallets, and actually bought the physical single –

or, later, paid to download a digital file.

That doesn't really work these days.

If you go back to 1952, when the charts were first launched,

a guy called Percy Dickens basically just used to get on the phone

and call up 20 of his mates who worked in record shops

and just ask them, what records were popular that particular week?

And it was as basic as that.

So for most of the history of the charts,

what we've done is we've counted sales,

because that's the way that consumers - music fans -

have been able to access their music.

Every year, hundreds of music acts release novelty Christmas singles,

hoping to be the one that makes it through the dreck

and gets to the top of the charts. And almost all of them fail,

although it's pretty heartwarming when they do succeed…

because it usually means that the winner of the X Factor

has been beaten down into second place,

and people seem to enjoy it when Simon Cowell doesn't win.

But calculating the numbers now is trickier than it used to be.

For the last 2½ years, we've been not just counting sales, but also adding streams in

there as well.

We convert the streams to sales by dividing the streams by 100,

and then we put it all together.

The 100 is essentially an estimate of the value of a stream versus the value of a download.

We're actually changing that from the beginning of 2017, to 150,

because what we're seeing is a massive shift

in terms of the number of streams that are being generated

and the number of sales that are being made as well.

Spotify, and Apple Music. Deezer, and Tidal.

Amazon have both Amazon Prime Music and Amazon Unlimited. We count those as well.

There are other, smaller services on top of that.

But pretty much any audio streaming service you'll be using, we count.

You'll notice something missing from that list of streaming services:

the official UK chart doesn't count YouTube views.

In theory, YouTube could track music use:

but it can't tell whether you're listening to a song

because you're there to listen to music…

or just because someone's decided to soundtrack their multi-million-view video

with Bing Crosby's White Christmas.

We've never counted video, and consumption of video, and we're not doing so now.

That said, obviously we're always talking about it,

it's a big debate within the industry, about whether it should or it shouldn't count.

That may change at some point in the future, but not quite for now.

I don't know who's going to be Christmas number one this year.

And neither do the folks here, at the official UK charts company,

at least as I record this,

although I suspect probably they've got a pretty good idea by now.

At 5:40pm on December 23rd,

BBC Radio 1 will make the announcement,

and – at least for this year –

someone will still be entering the history books.