Singers have been collaborating with each other since the invention of popular music, with many collaborations going on to be some of the best duets of all time.

Lady Gaga and Christina Aguilera sang a duet of Gaga's new song Do What U Want and released it on New Year's Day. Gaga previously featured R. Kelly on the song for her latest album Artpop.

Most popular duets have featured a male voice and a female voice, balancing each other throughout the song. Such duets include Diana Ross and Lionel Richie on Endless Love, Nelly and Kelly Rowland on Dilemma, Jordin Sparks and Chris Brown on No Air and Eminem and Rihanna for both Love the Way You Lie and The Monster.

Some duets have stood the test of time, such as Dolly Parton and Kenny Rogers' duet Islands in the Stream and Paul McCartney and Michael Jackson's duet Say Say Say.

Sometimes, the duet blends the voices together in a way that suits the purpose of the song. Such duets include Brandy and Monica's The Boy is Mine and Queen and David Bowie's Under Pressure.

Though many big artists collaborate with each other on a song or two, some decide to make full albums of duets, like Jay-Z and Kanye West with their duet album Watch the Throne.

Here is a gallery of some of the biggest duets in music history: