Its one or the other dude. The joint will be either 18.8mm or 14.5mm, unless you have a reducer. Downstem for 14.5mm joints Downstem for 18.8mm joints
It means that the downstem that came with the bong is a low profile diffuser; meaning that the downstem fits into a 18.8mm joint on a bong but the female part of the downstem(where the bowl inserts) is 14.5mm. In layman's terms, you should be buying/using 14.5mm bowls because 18.8mm bowls will be too big.
Thanks for the replies. Here's the item by the way: Percolator Bong Black - Black Leaf - English - Grasscity.com I was wondering because I wanted to buy an octopus (http://www.grasscity.com/pre-cooler-octopus-14-4.html) precooler, which comes in 14.5 and 18.8.
Exactly. If you are asking for my personal opinion I wouldn't recommend anything with 14.55mm.. it's just unnecessary to own both 14.55mm bowls and 18.8mms bowls especially if you plan to buy other bongs later. Just stick to 18.8mm everytime buying glass in my opinion.