The section of a water pipe that joins the bowl and main chamber. The downstem contributes to the provision of suction by drawing smoke or vapor into the water prior to inhalation. This long piece of glass can be fixed or taken off, and it can have a diffuser at the end of the chamber that also cools smoke.

The downstem, which connects the bowl to the water chamber, produces bubbles in the main chamber's water by pulling smoke through a bong or dab rig. The consumer then inhales vapor or smoke.
Downstems are either removable or can be attached to the bong or dab rig. They typically have thicknesses of 10mm, 12mm, or 14mm, and come in a variety of sizes. If you don't have the right size, you won't be able to draw smoke properly because there won't be an airtight seal between the main chamber and the bowl or nail.

Because they are simple to clean, removable downstems are the choice of the majority. They are frequently straight and can be easily scrubbed or quickly cleaned by submerging them in isopropyl alcohol. It is harder to get alcohol into fixed downstems, which typically have bends that make them harder to scrub.
Additionally, a glass water pipe with removable sections is more practical because, in the event that one of the sections, such as the downstem, breaks, you can simply replace that section rather than the entire pipe.