As many of you know Kulture VA is all about helping out our customers. We get questions all the time and want to post some of the ones we get more often on our site. For this post we will cover the following two questions. What does glass on glass mean and What are glass on glass joints? What does glass on glass mean? When a pipe is said to be glass on glass or “gong”, it means the stem and bowl use precision ground glass joints to make an airtight seal between them and the pipe as opposed to using rubber grommets. Ground joints were initially developed for laboratory use because of their airtightness, ease of use and non-reactivity. ROOR was the company that brought the use of ground glass joints to popularity in the pipe business. Within a decade of ROOR's innovation, nearly the entire pipe industry has embraced the glass joint as the preferred way to join pipe parts. What are glass on glass joints? Ground glass joints are conical shaped precision ground glass fittings that have a frosted surface. There are male and female versions and when fitted together they create a snug and airtight seal. The taper (1:10) on ground joints is the same universally no matter size or the manufacturer. Although the taper is universal, some manufacturers use different lengths. Typically the US made joints are shorter than the European made ones. In almost all cases, this does not effect fitment. This is a benefit to the glass collector, who could purchase a water pipe made by Illadelph, an Alex K shower head diffuser, a SYN ratchet perc ash catcher, and an AKM skull slide and have all items fit together. The most common sizes used in pipe making and 14mm and 18mm. Larger sizes like 24mm and 29mm are becoming more frequently seen used in pipe making. Sometimes you'll see joints listed as 18mm, 18.8mm or 19mm. What's the difference between 18mm, 18.8mm and 19mm joints? None, they are all 18.8mm joints. People tend to round up or round down the decimal. It is the same for 14mm and 14.4mm joints. The number is abbreviated for ease of use in conversation.