360 degree rotating hinge. Can be used to combine two parts that you need to rotate around each other.Print with no support - I used 20% fill and 3m thickness on a fast print. The space between cylinders is small enough that although the printer is printing in air it catches to build the top cylinder. You may need to use an exact-o knife to free it up a bit. I have printed this in 40mm and 20mm height and both worked fine.