R, Rp, Rc threads are taken from BS 21:1985 pipe threads for tubes and fittings where pressure tight joints are made on the threads.

G threads are taken from BS 2779:1986 pipe threads for tubes and fittings where pressure tight joints are not made on the threads. The pressure tight joint is achieved by the compression of a soft material (such as an o-ring seal or washer) between the end face of the male thread and a socket or nipple face, with a tightening of a back nut.