Two common strains of yeast are used to ferment beer: Saccharomyces carlsbergensis and Saccharomyces cerevisiae. Saccharomyces carlsbergensis, also known as bottom yeast, sinks to the bottom of the fermentation vat. Strains of bottom yeast ferment best at 42.8°F–53.6°F (6°–12°C) and take 8 to 14 days to produce lager beers. Saccharomyces cerevisiae, also known as top-fermenting yeast, is distributed throughout the wort and is carried to the top of the fermenting vat by the carbon dioxide (CO2). Top-fermenting yeast ferment at a higher temperature (57.2°F–73.4°F/14°–23°C) over only five to seven days. Top-fermenting yeasts produce ales, porter, and stout beers.