A male thread is an external thread.

A female thread is an internal thread.

A parallel or straight thread (commonly BSPP - British Standard Pipe Parallel thread) has a constant diameter and is denoted by the letter G.

A taper thread (commonly BSPT - British Standard Pipe Taper thread) has a diameter which increases or decreases along the length of the thread and is denoted by the letter R.

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.