British standard pipe (BSP) is a set of thread standards for pipes that have male and female ends for interconnection. There are two classes of BSP standards.
Parallel and Tapered
Parallel pipe threads are of equal size throughout the length of the thread. Tapered pipe threads change diameter from top to bottom. BSP standards for parallel threads are the "G" set of standards. BSP standards for tapered threads are the "R" set of standards. Parallel threaded taps are marked, "G Size ISO." Taper-threaded taps are marked "Rc Size ISO." There are no BSP standards for Rc tapered tap drills for 1.6 cm (5/8 inch), 2.2 cm (7/8 inch) or 4.4 cm (1 3/4 inch) thread.
There are 13 BSP parallel (BSPP) standards: 6.8mm for 1/16-inch thread; 8.8mm for 1/8-inch thread; 11.8mm for 1/4-inch thread; 15.25mm for 3/8-inch thread; 19mm for 1/2-inch thread; 21mm for 5/8-inch thread; 24.5mm tap drill for 3/4-inch thread; 28.25mm for 7/8-inch thread; 30.75mm for 1-inch thread; 39.5mm for 1 1/4-inch thread; 45.5mm for 1 1/2-inch thread; 51mm for 1 3/4-inch thread; 57mm for 2-inch thread.
There are 10 BSP tapered (BSPT) standards: 6.6mm for 1/16-inch thread; 8.6mm for 1/8-inch thread, 11.5mm for 1/4-inch thread; 15mm for 3/8-inch thread; 18.75mm for 1/2-inch thread; 24.25mm for 3/4-inch thread; 30.4mm for 1-inch thread; 39mm for 1 1/4-inch thread; 44.5mm for 1 1/2-inch thread; 56.75mm for 2-inch thread.