True Friends fans will know that Lisa Kudrow starred as Phoebe Buffay and her evil twin sister, Ursula.
True, TRUE, TRUUUUUEEEE Friends fans will know that Ursula, a crappy waitress, appeared in another popular 90s sitcom - Mad About You. At the same time.
Starring Helen Hunt and Paul Reiser, the show had no other links to Friends.
Kudrow had taken a job on Mad About You as a struggling young actress in need of a big break - before she had any idea how huge Friends would become.
Friends directors realised they needed to come up with a way to link the Mad About You universe and the Friends universe - and making Phoebe a twin was the perfect cover up.
That way, Friends fans wouldn't get confused about seeing the waitress from Mad About You turn up as masseuse Phoebe in Friends... because it was her sister, Ursula!
Kudrow said: "When Friends got its time slot on Thursday night at 8.30, after Mad About You, everybody realise 'we need some explanation for the audience - why are they seeing the same girl with the same voice at 8 o clock and 8.30 on two different shows?"
So there you have it folks! Watch the full video of Lisa explaining all: