3 Part Union Pipe Fitting Brass with BSP 3/4"threads. 

Brass 3 Part Union Pipe Fitting BSP 3/4" Threads  similar to a coupling, except it is designed to allow quick and convenient disconnection of pipes for maintenance or fixture replacement. While a coupling would require either solvent welding or being able to rotate all the pipes adjacent as with a threaded coupling, a union provides a simple nut transition, allowing easy release at any time.