Common tube fitting types and names

Tube fittings designed to connect a tube to pipe threads are called connectors. Tube fittings designed
to connect one tube to another are called unions:

If a tube union joins together different tube sizes rather than tubes of the same size, it is called
a reducing union.

A variation on the theme of tube connectors and unions is the bulkhead fitting. Bulkhead fittings
are designed to fit through holes drilled in panels or enclosures to provide a way for a fluid line to
pass through the wall of the panel or enclosure. In essence, the only difference between a bulkhead
fitting and a normal fitting is the additional length of the fitting “barrel” and a special nut used
to lock the fitting into place in the hole. The following illustration shows three types of bulkhead
fittings: