The Beatles at 50: From Fab Four to fabulously wealthy
Last May, in the US, the hit television series Mad Men made a little piece of history.
An episode of the 1960s period drama concluded with The Beatles' song Tomorrow Never Knows.
The song was not a cover version by another artist. It was the original tune.
As Mad Men's creator told the New York Times newspaper, no-one can remember a song written and recorded by The Beatles ever being licensed for use on a television series.
The producers of Mad Men reportedly paid about $250,000 (£155,000) for the recording and publishing rights to use the tune. Typically, licensing rights for major international pop songs sell for under $100,000.
The Mad Men deal underlines the enduring significance of a pop group that disbanded 42 years ago.
It also underlines the huge money-making potential that still resides in The Beatles' back catalogue.
After all these years, the band remain something of a money-making machine.
Just how much money will no doubt always remain a secret between those who control Beatles-world and the taxman.
But it is clear that the band continue to generate millions of pounds each year for those people and organisations with a stake in brand Beatles.
After the group split, the key issues became how to manage the name, the catalogue, and the complex finances.
Over subsequent decades, assets were shuffled between corporate, individual and family concerns.
There have been investments in non-music interests (some successful, some not), legal settlements, family disputes, and, of course, the deaths of two band members.
The main Beatles company is Apple Corp Ltd, controlled by Sir Paul McCartney, Ringo Starr and the estates of John Lennon and George Harrison.Artworks and property
Apple Corp administers the catalogue of Beatles releases, sales of which now total more than 600 million records, tapes and CDs. The firm holds the licensing rights for the brand and music, and has approval rights for the master recordings.
Apple recently oversaw, for example, the digitally remastered release of Yellow Submarine on DVD and CD.
According to the last accounts filed, Apple Corp paid £18m to its shareholders.
But there are other sources of wealth. Sir Paul was frequently referred to as the first music billionaire. But it seems that he is not there yet.
During his divorce in 2008 from Heather Mills, it was disclosed his assets were worth about £450m, and included art works by Picasso and Renoir, plus land and other property.
By 2012, according the annual Rich List compiled by the Sunday Times, his wealth had risen to £515m, up from £495m the year before.
Sir Paul still tours and records, and in 2011 his company grossed box office receipts of £50m. Another company, which handles some Lennon and McCartney song-writing revenues, made profits of £5.3m.
In addition, there was his huge post-Beatles success with his band, Wings.
Ringo Starr also still performs and records, with his All Starr band. Last year his company, Startling Music, paid him a £2m dividend, and the Sunday Times Rich List estimated his wealth at £160m.
Meanwhile, the widow of George Harrison, Olivia, and their son Dhani, had a joint fortune of £180m, according to the Rich List.
As for John Lennon, much of his estate is controlled via the US and administered by Yoko Ono. The estate's value is reportedly close to £200m.
The beneficiaries of The Beatles' legacy go far beyond Apple Corp, however. Sony ATV holds the rights to Beatles music and lyrics, and EMI Group controls the master recordings.
The proposed $1.9bn sale of EMI to Universal Music announced in September only got approval from competition regulators after EMI agreed to sell about a third of its assets to other music companies.
Selling off The Beatles, though, was most certainly not part of the deal. They continue to be far too valuable.
The complicated "ownership" of The Beatles was blamed by some for holding back the full exploitation of the band.
Things are changing, though, and over the past few years new revenue streams - big and small - have opened up.Lennon ice cream
The Beatles were slow to the online music revolution, perhaps because they were profiting so hugely from CD sales.
Also, with The Beatles having pioneered the concept album, there was a reluctance by some involved in managing the legacy to see the music broken up into downloadable bites.
However, Beatles songs finally became available on iTunes in November 2010.
Within a couple of days, 28 of the top 100 tunes being downloaded were by The Beatles. The band also made up 16 of the top 50 albums, including four in the top 10.
There have also been anthology projects, a tour by the Cirque du Soleil circus group called The Beatles Love, and The Beatles Rock Band video game.
Beatles ring tones, Monopoly and Trivial Pursuit games, and a Ben & Jerry's ice cream called Imagine Whirled Peace - in honour of Lennon - also provide a financial cut for the estate.
But it is the music that is the cash cow, and there are few signs that it is going out of favour.
Rather, The Beatles seem to have transcended the divide between young and old, the fashionable and unfashionable.
So it's a fair bet that each new generation will continue to buy the re-packaged, re-mastered, re-heated music of the world's most successful band.