There are two main types of yeast, ale and lager. Ale yeasts are referred to as top-fermenting because much of the fermentation action takes place at the top of the fermenter, while lager yeasts would seem to prefer the bottom. While many of today's strains like to confound this generalization, there is one important difference, and that is temperature. Ale yeasts like warmer temperatures, going dormant below about 55°F (12°C), while lager yeasts will happily work at 40°F. Using certain lager yeasts at ale temperatures 60-70°F (18-20°C) produces a style of beer that is now termed California Common Beer. Anchor Steam Beer revived this unique 19th century style.