These fittings are made to tighter tolerances than other stainless steel pipe fittings. They are also known as instrumentation fittings.

Socket-connect fittings are easier to weld than butt-weld fittings because the fitting doesn't need to align perfectly to the pipe. Slip the pipe into the socket and weld.

Pipe and fittings weld together for a permanent connection that is stronger and less likely to leak than threaded and clamp-on connections.