Because of the taper, a pipe can only screw into a pipe fitting a little distance before it jams.
Plumbing Union Fitting
Pipe Union:Hex Head Socket Weld Union and Thread Union
Halfords Brake Pipe Unions Metric Male (FIXG290)
Union Pipes and Fittings, CPVC Union
SS304 Pipe Union Connector 1/4-2 BSPT Female-Male Threaded
in Pipe Fittings.
4Pcs 1 PVC Union Coupling Pipe Fitting (Socket x Socket) 1Inch PVC Slip Union PVC Adapter Coupling Schedule 40, EPDM O-Ring, White
Copper Pipe Fittings - Union, MISUMI
62-4 COMPRESSION UNION COUPLING 1/4T 884.62-4
3/4 Copper Union