Bock is a dark larger- which is interesting because most dark beers are ales. It is a seasonal beer, usually made in the later winter and early spring. The myth is that bock is made in the spring when the biermeister cleans out the vats and uses the sludge at the bottom to make bock. Some bartender's guides even perpetuate this myth. In reality, bock is made freshly just like every other beer. For one thing, the essence of any fermentation process is a strictly controlled, highly sanitized environment to keep unwanted bacteria from fouling the beer, so brewing with leftover gunk would be an abomination to any professional brewer. Secondly, the residue at the bottom of a fermentation vat is useless- the yeast has died and there is virtually no digetible carbohydrate in it that fresh yeast can eat.