Short definition
A plug is a male-threaded closure for the end of a pipe or fitting — the opposite of a cap, which is female and slips over a pipe end. Used in threaded systems (galvanized, black iron, brass) to close off a tee outlet, terminate a pipe end, or seat into a cleanout fitting. Available in square-head, hex-head, and recessed-socket styles.
What it is
A plug threads into a fitting. You’ll see them in three common spots: at unused tee outlets in older threaded systems, at DWV cleanouts where the plug is removed periodically for maintenance, and at gas test ports where a plug seals the pipe during pressure testing.
Why it matters to a homeowner
The most common homeowner encounter is a cleanout plug — the square-head or recessed-socket plug that threads into a DWV cleanout fitting. To snake a clogged drain, you remove this plug. Re-installing requires PTFE tape or pipe dope to seal cleanly.
Common variants and what a plug is not
- Plug vs. cap. Plug is male (threads into a fitting); cap is female (slips over a pipe end).
- Pipe plug vs. cleanout plug. Same fitting class, with the cleanout plug specifically the one used at code-required DWV cleanout locations.