There are various types of compression fittings in the world. Today the brass pipe compression fitting we would talk about is used in the tube for water or gas system.
The pipe compression fittings are one of the brass pipe fittings. It can connect 2 pipes or a pipe with a valve with compression type nipple. It is assembled by the brass fitting body with compression seats, compression nuts, and brass rings.
When the compression nut is tightened, the ring causes the pressure against the pipe and the nuts and seals the connector, providing an absolutely tight connection.