A slip joint extension tube is used to make residential plumbing easier. Rather than requiring all the piping under a sink to be measured, cut, and threaded exactly to length, the slip joint extension tube slides into another piece of pipe and seals with a plastic ring. This allows a few inches of slop in the installation, simplifying matters greatly. They typically come in 1-1/2 inch and 1-1/4 inch outer diameter sizes, and 6 inch and 12 inch lengths.