Dubbel
The term dubbel (also double) is a Belgian Trappist beer naming convention. The origin of the dubbel was a strong version of a brown beer brewed in Westmalle Abbey in 1856, which is known to have been on sale to the public by June 1861. In 1926, the recipe was changed, and it was sold as Dubbel Bruin. Following World War Two, abbey beers became popular in Belgium and the name "dubbel" was used by several breweries for commercial purposes. Westmalle's Dubbel was imitated by other breweries around the world, both Trappist and secular, leading to the emergence of a style.
The term dubbel (also double) is a Belgian Trappist beer naming convention. The origin of the dubbel was a strong version of a brown beer brewed in Westmalle Abbey in 1856, which is known to have been on sale to the public by June 1861. In 1926, the recipe was changed, and it was sold as Dubbel Bruin. Following World War Two, abbey beers became popular in Belgium and the name "dubbel" was used by several breweries for commercial purposes.
Westmalle's Dubbel was imitated by other breweries around the world, both Trappist and secular, leading to the emergence of a style. Dubbels are now understood to be a fairly strong (6–8% alcohol by volume) brown ale, with understated bitterness, fairly heavy body, and a pronounced fruitiness and cereal character. 
Chimay Première (Red), Koningshoeven/La Trappe Dubbel, and Achel 8 Bruin are notable examples from Trappist breweries. Affligem and Grimbergen are Belgian abbey breweries that produce dubbels.
Notable examples from the US include Ommegang's Dubbel and New Belgium's Abbey Ale.
Abbey 1856 Dubbel is produced in Argentina.

Flavour profile
Dubbels are characteristically known for being dark brown in colour with a strong flavour of dark fruit including raisins, prunes, and dates. These flavours and colours are almost entirely resultant from the heavy addition of highly caramelized (or kilned) beet sugar, which ferments completely into alcohol, lightening the body of the finished beer and contributing to its dry finish. The caramelization of the beet sugar is also the major contributor of maillard flavors including chocolatey, caramel, and nutty tones that give the dubbel its wide gamut of flavour complexity. Because of the special strains of ale yeast used in their production, dubbels often carry a mild spice; coriander and black pepper are notable examples in traditional Belgian dubbels.

See also
Tripel
Quadrupel
Trappist beer
Beer in Belgium

References
External links
Belgium's Great Beers
A history and guide to Trappist breweries in Belgium
Another history and guide to Trappist breweries in Belgium
The Belgian Beer Pub Map