Most smokers know how effective a tree perc can be in your bong. Now imagine having a tree perc at the end of your downstem. The 18mm to 14mm Tree Perc Downstem by DankStop allows your smoke to be filtered when it leaves the downstem. With 6 arms, this downstem diffuses your smoke plenty before it reaches your lips. Choose the size that fits your bong best. Having trouble finding out which length you need? Use our easy measuring guide.