The Brewers Association (US) offers the following style parameters:
Traditional German-Style Bock
"Traditional bocks are made with all malt and are strong, malty, medium- to full-bodied, bottom-fermented beers with moderate hop bitterness that should increase proportionately with the starting gravity. Hop flavor should be low and hop aroma should be very low. Bocks can range in color from deep copper to dark brown. Fruity esters should be minimal."
Original Gravity (ºPlato): 1.066-1.074 (16.5-18 ºPlato)
Apparent Extract/Final Gravity (ºPlato): 1.018-1.024 (4.5-6 ºPlato)
Alcohol by Weight (Volume): 5-6% (6-7.5%)
Bitterness (IBU): 20-30
Color SRM (EBC): 20-30 (40-60 EBC)
The BJCP offers the following style parameters:
5B. Traditional Bock
Aroma: "Strong malt aroma... Virtually no hop aroma. Some alcohol may be noticeable."
Appearance: "Light copper to brown color, often with attractive garnet highlights."
Flavor: "Complex maltiness is dominated by the rich flavors of Munich and Vienna malts, which contribute melanoidins and toasty flavors. Some caramel notes may be present from decoction mashing and a long boil."
Mouthfeel: "Medium to medium-full bodied. Moderate to moderately low carbonation. Some alcohol warmth may be found, but should never be hot. Smooth, without harshness or astringency."
Overall Impression: A dark, strong, malty lager beer.
History: "Originated in the Northern German city of Einbeck, which was a brewing center and popular exporter in the days of the Hanseatic League (14th to 17th century)... The name “bock” is based on a corruption of the name “Einbeck” in the Bavarian dialect..."
OG: 1.064 – 1.072
IBUs: 20 – 27
FG: 1.013 – 1.019
SRM: 14 – 22
ABV: 6.3 – 7.2%