Plumbing incorporates many different types of fittings. These range from copper solder (end feed) compression (brass) solder ring (pre-soldered/Yorkshire) solvent weld, and push fit. Generally speaking, standard fittings will range from 8mm up to 28mm, but there are larger sizes available depending on the application.
Joints are made to connect pipes at different angles and in various combinations; there are adapters for joining metric and imperial pipe work, and for connecting one kind of a material to another. Plumbing fittings such as valves are made with de-mountable compression joints, so that they can be easily removed for servicing or replacement.
Metric/Imperial Adapters
Copper pipes are now made in metric sizes, whereas pipe work installed in an older house will have been made to imperial measurements, if you compare 15mm (1/2in) and 22mm (3/4in) the difference would be obvious, but metric pipe is measured externally, whereas imperial is measured internally, hence the need in some cases for metric/imperial adapters.
Solvent weld or waste fittings are made from MuPVC and are measured from 32mm (1 1/4in) and 40mm (1 1/2in) upwards; this type of pipe work is fused together by means of a solvent-based glue.
Typically, 15mm (1/2in) pipe is used to supply basins, kitchen sinks, washing machines, some shower application, and radiator flow & returns.
However 22mm (3/4in) pipe is used to supply baths, high output showers, hot water cylinders and main central heating.