An elbow is a pipe fitting installed between two lengths of pipe or tubing to allow a change of direction, usually a 90° or 45° angle, though 22.5° elbows are also made. The ends may be machined for butt welding, threaded (usually female), or socketed, etc. When the two ends differ in size, the fitting is called a reducing elbow or reducer elbow.

Elbows are design features as below

A 90 degree elbow is also called a “90 bend” or “90 ell”. It is a fitting which is bent in such a way to produce 90 degree change in the direction of flow in the pipe. It used to change the direction in piping and is also sometimes called a “quarter bend”.

A 90 degree elbow attaches readily to plastic, copper, cast iron, steel and lead. It can also attach to rubber with stainless steel clamps. It is available in many materials like silicone, rubber compounds, galvanized steel, etc. The main application of an elbow (90 degree) is to connect hoses to valves, water pressure pumps, and deck drains. These elbows can be made from tough nylon material or NPT thread.

A 45 degree elbow is also called a “45 bend” or “45 ell”. It is commonly used in water supply facilities, food industrial pipeline networks, chemical industrial pipeline networks, electronic industrial pipeline networks, air conditioning facility pipeline, agriculture and garden production transporting system, pipeline network for solar energy facility, etc.

Elbows are split into two groups which define the distance over which they change direction; the center line of one end to the opposite face. This is known as the "center to face" distance and is equivalent to the radius through which the elbow is bent.

Here below, for example, you will find the center to face distance of NPS 2 elbows (the A distance on the image)

The center to face distance for a "long" radius elbow, abbreviated LR always is "1? x Nominal Pipe Size (NPS) (1?D)", while the center to face distance for a "short" radius elbow, abbreviated SR even is to nominal pipe size.