A bong without a downstem is just a flower vase.
Pretty useless towards your cause, right? A downstem is basically the heart of every glass piece that fits into a water pipe joint. Essentially it makes it all cycle. Downstems function to carry smoke from the bowl (aka slide) into the water chamber of your glass water pipe, bong, dab rig to filter and cool each hit. Downstems are available in a variety of different sizes and lengths, so it is absolutely essential to know what size you need before purchasing one.