When it comes to plumbing, it's important to understand the different types of threads used in pipes and fixtures. In the United States, water pipes and supply lines are typically threaded with a conical thread on the right. However, some plumbing fixtures have reverse threads to prevent incorrect lines from being mistakenly connected. Reverse threads are also used for gases such as oxygen and acetylene, so that the hoses cannot mix and cause a fire or explosion.
These reverse threads usually come in four lengths and are used for gas pipes when joints are not allowed. If you need to cut a reverse thread, most plumbing supply houses should have dies available, although it may be more expensive than expected. When it comes to water heaters, a dielectric connection is needed from the steel tank if copper tubing is used. It's important to not tighten the fittings too tight, as this can strip the threads inside the fitting or on the outside of the pipe.
The same principle applies when turning a screw, opening a jar or bottle, operating a faucet, and adjusting a plumbing fixture. Garden hoses are also designed not to interconnect with normal pipes to prevent any accidents.