Porter is said to have been popular with transportation workers of Central London, hence the name. Most traditional British brewing documentation from the 1700s state that Porter was a blend of three different styles: an old ale (stale or soured), a new ale (brown or pale ale) and a weak one (mild ale), with various combinations of blending and staleness. The end result was also commonly known as “Entire Butt” or “Three Threads” and had a pleasing taste of neither new nor old. It was the first truly engineered beer, catering to the public’s taste, playing a critical role in quenching the thirst of the UKs Industrial Revolution and lending an arm in building the mega-breweries of today.
Porter saw a comeback during the homebrewing and micro-brewery revolution of the late 1970s and early 80s, in the US. Modern-day Porters are typically brewed using a pale malt base with the addition of black malt, crystal, chocolate or smoked brown malt. The addition of roasted malt is uncommon, but used occasionally. Hop bitterness is moderate on the whole and color ranges from brown to black. Overall they remain very complex and interesting beers.
Porters of the late 1700’s were quite strong compared to todays standards, easily surpassing 7% alcohol by volume. Some brewers made a stronger, more robust version, to be shipped across the North Sea, dubbed a Baltic Porter. In general, the styles dark brown color covered up cloudiness and the smoky/roasted brown malts and bitter tastes masked brewing imperfections. The addition of stale ale also lent a pleasant acidic flavor to the style, which made it quite popular. These issues were quite important given that most breweries were getting away from pub brewing and opening up breweries that could ship beer across the world. (BeerAdvocate)