Trappist

Trappist beer is brewed by Trappist monks. Twelve monasteries—six in Belgium, two in the Netherlands, and one each in Austria, Italy, England and the United States—currently brew Trappist beer.
 
Trappist beers are mostly top-fermented and mainly bottle conditioned. Trappist breweries use various systems of nomenclature for the different beers produced which relate to their relative strength.
 
The best known is the system where different beers are called Enkel/Single, Dubbel/Double, Tripel/Triple and Quadrupel/Quadruple. These terms roughly describe both the amount of malt and the original gravity for a Dubbel and triple for a Tripel, marking the casks accordingly.
 
Colours can be used to indicate the different types, dating back to the days when bottles were unlabelled and had to be identified by the capsule or bottle-top alone. Chimay beer labels are based on the colour system (in increasing order of strength red, white and blue). Westvleteren beers are still unlabelled.
 
You can read more about Trappist beer at https://en.wikipedia.org/wiki/Trappist_beer