By BEERMAG —
At first glance, those numbers might seem to refer to alcohol content, but the answer is not that straightforward. Historically, Belgian monks in Trappist monasteries and abbeys brewed beers using a system called Belgian degrees.
The Belgian degree is a measurement used to determine how much sugar is in an unfermented beer. By knowing the quantity of sugar in the beer before and after fermentation, the brewer can determine the amount of alcohol produced. To understand what a Belgian degree is, one has to understand what the specific gravity of a beer is. When you dissolve sugar into a solution, the liquid becomes denser, causing a calibrated floating device called a hydrometer to float higher. The measurement given by a hydrometer is called specific gravity. The specific gravity of water is said to be 1.000. By adding sugar from malted barley and other fermentables, the specific gravity increases. An original specific gravity of 1.060 would yield a beer with a Belgian degree number of 6. You obtain this number by subtracting 1 from 1.060 and multiplying by 100. So a beer with a gravity of 1.080 would measure 8 Belgian degrees, and a beer with a gravity of 1.120 would yield 12.
Beers like Westvleteren XII originally had an original gravity of 1.120, which produced a beer in the 12% ABV range. Today, the original gravity is closer to 1.086, and the alcohol is closer to 10.5% ABV, yet they still label the beers according to the old-school Belgian degrees number.