Before you purchase a downstem, there are three things you need to confirm in order to choose the correct downstem for your bong. Firstly, confirm your bong's joint size (10mm, 14mm or 18mm). This is the joint where your downstem is inserted into the bong. Measure the width of your bong's joint which should match the widest part of your downstem (the top of the frosted glass that fits flush in your bong's joint). Secondly, confirm the downstem length by measuring from the top of the downstem (the top is where your cone piece is inserted) to the bottom (where it sits in the water chamber). Lastly, ensure the downstem's joint will fit your cone piece that you're inserting into it (10mm, 14mm or 18mm).