Stem for 18mm to 14mm
A Down Stem is a piece of scientific glass that connects a tube-style water pipe to its slide bowl. It's easy to break and can get quite dirty so people like to have replacement options.
This particular down stem has a 18mm male fitting and 14mm female fitting. It's made to fit into a water pipe with a 18mm opening.
It leads up to a female fitting with an 14mm opening.