A Homeowner's Reference Guide to Common Plumbing Fittings

Why Knowing Your Fittings Matters

Whether you're describing a leak to a plumber over the phone or picking up a part at the hardware store, using the correct terminology saves time and prevents costly mistakes. Plumbing fittings are the connectors that join pipes, change their direction, split flow, or change pipe diameter. Residential systems typically use fittings made from copper, CPVC, PVC, PEX, or galvanized steel — and each material has fittings designed specifically for it. This guide covers the most common fitting types you'll encounter in a standard home.

Note: This article is for general educational purposes. Always consult a licensed plumber before undertaking any work that requires permits, involves opening walls, or affects supply lines and drain stacks. Check local building codes for requirements in your area.

The Core Fitting Types Explained

Couplings

A coupling connects two pipes of the same diameter in a straight line. Standard couplings require cutting the pipe; slip couplings (also called repair couplings) can slide over existing pipe to bridge a damaged section without disassembling the full line — a useful distinction when making repairs.

Elbows

An elbow changes the direction of a pipe run. The two most common angles are 90-degree elbows (a sharp turn) and 45-degree elbows (a gentler angle that reduces flow resistance). A street elbow has one male and one female end, allowing it to connect directly to another fitting without a short pipe nipple between them.

Tees

A tee fitting is T-shaped and splits or combines flow at a branch point. The straight-through ports are called the run; the perpendicular port is the branch. A sanitary tee, used in drain-waste-vent (DWV) systems, has a curved interior that guides wastewater in the correct direction to prevent clogging and siphoning.

Reducers and Bushings

A reducer (or reducing coupling) connects pipes of two different diameters, stepping flow down from a larger to a smaller pipe or vice versa. A bushing does the same job but fits inside an existing fitting rather than between two pipe ends, making it a compact alternative when space is tight.

Unions

A union joins two pipes like a coupling but can be disassembled without cutting the pipe — it has a threaded nut that holds the two halves together. Unions are typically installed near water heaters, pumps, and valves to allow future maintenance without tearing out the entire line.

Caps and Plugs

A cap fits over the outside of a pipe end to seal it; a plug threads or fits inside a fitting to seal it. Both are used when capping off an unused branch or pressure-testing a system.

Material and Thread Basics

Fittings must match both the pipe material and the thread type. The two most common thread standards in U.S. residential plumbing are NPT (National Pipe Taper), which creates a pressure-tight seal as threads tighten, and NPS (National Pipe Straight), which requires a gasket or O-ring to seal. Always confirm thread type before purchasing threaded fittings.

Mixing metals — such as connecting copper pipe directly to galvanized steel — can cause galvanic corrosion, a chemical reaction that degrades both materials over time. A dielectric union or dielectric nipple is designed specifically to separate dissimilar metals and prevent this reaction.

For plastic systems, solvent welding (using PVC or CPVC cement) creates a chemical bond rather than a mechanical one. PEX pipe uses crimp, clamp, or expansion fittings that require dedicated tools. Understanding these distinctions helps you communicate clearly with a plumber and recognize when a repair is within DIY scope versus when it warrants professional help — much like learning the terminology in any technical field. (If you find reference guides like this one useful, our Car Care A–Z glossary applies the same plain-English approach to vehicle maintenance terms.)