Read this: Stout and porter are dark beers made using roasted malts or roast barley, and typically brewed with slow fermenting yeast. There are a number of variations including Baltic porter, dry stout, and Imperial stout. The name Porter was first used in 1721 to describe a dark brown beer popular with the street and river porters of London. This same beer later also became known as stout, though the word stout had been used as early as 1677. The history and development of stout and porter are intertwined.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What type of yeast is used to make stout and porter beers?
slow fermenting yeast