Just as regularly as the discussions about the difference between porter and stout appear on beer forums, someone quotes the Beer Judge Certification Program (BJCP) guidelines that identify the difference between the two as being the roasted malt employed: black malt for porter, roasted barley for stout. Unfortunately, the guidelines were written centuries after the fact and ignore how porter and stout were originally brewed.
To get to the root of the question, we have to travel back in time a couple of hundred years to the beginning of the eighteenth century, the time when porter and stout emerged as specific styles. In the classifications of the day, there were two types of malt liquor brewed: lightly hopped ale and heavily hopped beer. These were further subdivided by the base malt employed (pale, amber, or brown) and strength (small, common, and stout). The first porters and stouts were classified as types of brown beer—common brown beers and stout brown beers, respectively—meaning that they were well hopped and brewed from 100 percent brown malt.
A decade or so ago, I spent much time pondering the meaning of the term “brown stout.” Why did some brewers call their stouts by this name? The answer was stunningly simple: Until the nineteenth century, brown stout wasn’t the only type of stout. There was also pale stout, a beer of similar strength brewed from pale malt. Stout meant nothing more than “strong,” and the brown qualifier was needed to tell you what type of beer to expect.