Piping and plumbing fitting

Fittings (especially uncommon types) require money, time, materials and tools to install, and are an important part of piping and plumbing systems. Valves are technically fittings, but are usually discussed separately. 

The material with which a pipe is manufactured is often the basis for choosing a pipe. Materials used for manufacturing pipes include:
Carbon (CS) and galvanized steel
Impact-tested carbon steel (ITCS)
Low-temperature carbon steel (LTCS)
Stainless steel (SS)
Malleable iron
Non-ferrous metals (includes copper, inconel, incoloy and cupronickel)
Non-metallic (includes acrylonitrile butadiene styrene (ABS), fibre-reinforced plastic (FRP), polyvinyl chloride (PVC), high-density polyethylene (HDPE) and toughened glass)
Chrome-molybdenum (alloy) steel — Generally used for high-temperature service
The bodies of fittings for pipe and tubing are most often the same base material as the pipe or tubing connected: copper, steel, PVC, chlorinated polyvinyl chloride (CPVC) or ABS. Any material permitted by the plumbing, health, or building code (as applicable) may be used, but it must be compatible with the other materials in the system, the fluids being transported and the temperature and pressure inside (and outside) the system. Brass or bronze fittings are common in copper piping and plumbing systems. Fire hazards, earthquake resistance and other factors also influence the choice of fitting materials. 
 Two threaded adapters for connecting copper pipe (sweat) to a female thread
In plumbing, an adapter is generally a fitting which interfaces two dissimilar parts. The term commonly refers to:
any fitting that connects pipes of different materials, including:
expansion adapters which have a flexible section to absorb expansion or contraction from two dissimilar pipe materials
mechanical joint (MJ) adapters for joining PE pipe to another material
bell adapters which are like mechanical joint adapters but contain a stainless steel backup ring to maintain a positive seal against the mating flange
flange adapters which attach to a PE pipe with butt fusion to stiffen a junction and allow another flanged pipe or fitting to be bolted on
a fitting that connects pipes of different diameters, genders, or threads (see § Coupling below)
adapter spools (also called crossover spools), used on oilfields and pressure control, have different diameters, pressure ratings or designs at each end
adapters to convert NPT to BSP pipe threads are available
a fitting that connects threaded and non-threaded pipeSee also: Street elbow
 Short-radius (or regular) 45° elbow (copper sweat)
 Long-radius (or sweep) 90° elbow (copper sweat)
An elbow is installed between two lengths of pipe (or tubing) to allow a change of direction, usually a 90° or 45° angle; 22.5° elbows are also available. The ends may be machined for butt welding, threaded (usually female), or socketed. When the ends differ in size, it is known as a reducing (or reducer) elbow.
A 90º elbow, also known as a “90 bend”, “90 ell” or “quarter bend”, attaches readily to plastic, copper, cast iron, steel, and lead, and is attached to rubber with stainless-steel clamps. Other available materials include silicone, rubber compounds, galvanized steel, and nylon. It is primarily used to connect hoses to valves, water pumps and deck drains. A 45º elbow, also known as a “45 bend” or “45 ell”, is commonly used in water-supply facilities, food, chemical and electronic industrial pipeline networks, air-conditioning pipelines, agriculture and garden production, and solar-energy facility piping.
Elbows are also categorized by length. The radius of curvature of a long-radius (LR) elbow is 1.5 times the pipe diameter, but a short-radius (SR) elbow has a radius equal to the pipe diameter. Short elbows, widely available, are typically used in pressurized systems, and in physically tight locations.
Long elbows are used in low-pressure gravity-fed systems and other applications where low turbulence and minimum deposition of entrained solids are of concern. They are available in acrylonitrile butadiene styrene (ABS plastic), polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), and copper, and are used in DWV systems, sewage, and central vacuum systems.
Main article: Coupling (piping)
 Pipe coupling (copper sweat)
A coupling connects two pipes. If their sizes differ, the fitting is known as a reducing coupling, reducer, or an adapter. There are two types of couplings: “regular” and “slip”. A regular coupling has a small ridge or stop internally, to prevent over-insertion of a pipe, and thus under-insertion of the other pipe segment (which would result in an unreliable connection). A slip coupling (sometimes also called a repair coupling) is deliberately made without this internal stop, to allow it to be slipped into place in tight locations, such as the repair of a pipe that has a small leak due to corrosion or freeze bursting, or which had to be cut temporarily for some reason. Since the alignment stop is missing, it is up to the installer to carefully measure the final location of the slip coupling to ensure that it is located correctly.
 Combination union and reducer (brass threaded)
A union also connects two pipes, but is quite different than a coupling, as it allows future disconnection of the pipes for maintenance. In contrast to a coupling requiring solvent welding, soldering, or rotation (for threaded couplings), a union allows easy connection and disconnection, multiple times if needed. It consists of three parts: a nut, a female end and a male end. When the female and male ends are joined, the nut seals the joint by pressing the two ends tightly together. Unions are a type of very compact flange connector.
Dielectric unions, with dielectric insulation, separate dissimilar metals (such as copper and galvanized steel) to prevent galvanic corrosion. When two dissimilar metals are in contact with an electrically conductive solution (ordinary tap water is conductive), they form an electrochemical couple which generates a voltage by electrolysis. When the metals are in direct contact with each other, the electric current from one to the other also moves metallic ions from one to the other; this dissolves one metal, depositing it on the other. A dielectric union breaks the electrical path with a plastic liner between its halves, limiting galvanic corrosion.
Rotary unions allow mechanical rotation of one of the joined parts, while resisting leakage.