Porter was brewing’s first rock star beer style. First emerging in London in the early 1720s, the popularity of this dark brown, roasted ale transformed the nature of commercial brewing from a trade into an industry. By 1800 it had spawned many forms and was exported around the globe.
Experts differ about whether there are or should be definable differences between a Porter and a Stout. The latter word had long been used in brewing to mean a stronger beer but by 1750 was synonymous with stronger Porters and by 1800 on mainland Britain the types of beer named Stout and Porter were indistinguishable, though in other places, not least the island of Ireland, they continued to be used separately for different styles, though not always for consistent reasons.
Conventions invented since the gradual revival of Porter since 1975 cite it is lighter in alcohol and colour, and perhaps fruitier, with Stout more often featuring roasted barley, though historical precedent for this rarely survives examination.
One sub-style we refuse to list other than as a warning is Oyster Stout. Different stories of its origins have it as a 20th century brew designed for pairing with freshly shucked oysters; a beer of Victorian origin rendered dry by filtering through crushed discarded shells; or either a true story or inspired hoax from New Zealand in the 1920s that had it brewed with shellfish as an ingredient. Whether it should be sweet or dry depends on your preferred truth.