A Mate-Demate Device is a specialized crane designed to lift a Space Shuttle orbiter

Space Shuttle Orbiter

The Space Shuttle orbiter was the orbital spacecraft of the Space Shuttle program operated by NASA, the space agency of the United States. The orbiter was a reusable winged "space-plane", a mixture of rockets, spacecraft, and aircraft...

 onto and off of the back of a Shuttle Carrier Aircraft

Shuttle Carrier Aircraft

The Shuttle Carrier Aircraft are two extensively modified Boeing 747 airliners that NASA uses to transport Space Shuttle orbiters...

. Three Mate-Demate Devices were built, all of different designs.

The first MDD is located at Edwards Airforce Base in California. Many Space Shuttle missions were not able to land in Florida due to bad weather. All but one (STS-3

STS-3

STS-3 was NASA's third Space Shuttle mission, and was the third mission for the Space Shuttle Columbia. It was the first shuttle launch with an unpainted external tank, and the only mission to land at the White Sands Space Harbor near Las Cruces, New Mexico.-Crew:-Backup crew:-Mission...

) of these flights landed at Edwards instead. This MDD was used to hoist a Space Shuttle orbiter onto a Shuttle Carrier Aircraft for the flight back to Florida.

The second MDD is located at the Kennedy Space Center in Florida. It's primary use is unloading the orbiter after its cross-country flight from Edwards.
The third MDD was built for Space Shuttle operations at Vandenberg Airforce Base in California.

On rare occasions an orbiter needs to be loaded or unloaded at a location where no MDD is available. In these cases a pair of cranes is used instead.