Most places have got one. Some even have several. But many people know nothing about them.
Twin towns supposedly encourage tourism, help us experience new cultures and bring us closer to places all around the world.
Nottingham, Derby and Leicester all have German partners thanks to efforts by Germany to improve its relationship with Britain after World War II.
But twinning can be seen as outdated, expensive or even pointless - so is it still relevant in 2013?
Love it or hate it?
On a recent trip to Germany, BBC East Midlands Today reporter Tom Brown found a thriving community of people who love the idea of twinning.
One example is Leicester's relationship with Krefeld, a town about 16 miles (27km) northwest of Dusseldorf.
The cities' two fire services have struck a friendship over football. They have played each other every year since 1973.
Perhaps surprisingly, this year the English team won on penalties.
There are also countless examples of cultural links. Theatres, choirs and orchestras across the East Midlands often perform in their towns' twin.
But in this era of austerity, twinning is not just about making friends, but making money.
When it comes to trade, Nottingham seems to be leading the way.
Several companies have benefited from trade missions to its German partner, Karlsruhe, including ocularist John Pacey-Lowrie, who exports acrylic, prosthetic eyes to Germany.
He said Nottingham's links with Karlsruhe had definitely helped him make inroads into a foreign market.
"They (Germans) are very aware of the links with Nottingham and the Germans particularly like that sort of thing.
"I'd never thought before about twinning and why we do it but it's really good and it's great for commerce."
Nottingham's desire to turn twinning links into business links is beginning to make waves around the rest of the region.
"They [Nottingham and Karlsruhe] are pretty much leading the pack," said the regional director of UK Trade and Investment, Peter Hogarth.
"There is a will on both sides to try and put a commercial level on to an existing relationship. In doing that, you can get a real benefit for business."
But Nottingham's relationship with its German twin goes further than just trade links.
Karlsruhe also boasts one of the most extensive and efficient tram networks in Germany. Given the country's lauded public transport infrastructure, this is certainly saying something.
Back in the 1990s, Nottingham asked Karlsruhe for advice on restoring its tram network.
Experts from Germany visited the city to help set a new one up. Now, in 2013 Nottingham is completing its second tram line.
The East Midlands city's gratitude is shown by the name of the new tram bridge at the station: the Karlsruhe Friendship Bridge.
Price of friendship
But twin towns do not come free of charge. Derby City Council has an annual twinning budget of £35,000.
It uses part of that to employ an envoy. This year, 22-year-old Rob Bentley is Derby's ambassador and is spending the year in the city's German twin, Osnabrück.
In return, the German city will send an envoy back to the East Midlands.
"Obviously twinning is an expense", Mr Bentley said, "But it pays for itself.
"There are so many students who come over from Osnabrück to do work placements in Derby. It's just money that wouldn't be in the local economy otherwise."
Leicester City Council - which needs to save £61m by 2018 - has spent £100,000 on its five twinning links in the past three years. That included £1,661.30 on flags.
The chairman of the City of Leicester European Twinning Association, Peter Lee, said the city did get good value for money, but it could do more.
"What we really need, in order to get an economic benefit, trade and that sort of thing, would be full-time workers on the project," said Mr Lee.
"I don't think there's anywhere near enough trade between Leicester and Krefeld."
But more subtle than the trade links are the friendships that have been borne out of the twinning projects.
In 2002, when Leicester firefighter Bob Miller died in a factory fire, Krefeld firefighter Ulf Tabbart not only got a group together to go to the funeral but also organised a fundraising event in his honour.
"Friendship is like the stars in the sky - you cannot see them but you know they are there."