There are all manner of theories and logical gymnastics employed today by brewers, beerophiles, and even the BJCP alike in attempts to differentiate the "styles" of stout and porter, both of which were not really classified separately during their heyday, and only pigeonholed after their recent revival beginning in the 1970s. We do know that the designation of "porter" came about between 1720 and 1730, with the first mention being in a letter from a Frenchman to his family in 1726, and the first official porter brewed in 1730 by Ralph Harwood of the Bellin Shoreditch in London. After the popularity of porters began to rise, the designation of "stout" began to be put to use for both porters and pale ales, and simply meant a stronger version of the beer. Indeed, even up until the 1840s, one could have strolled into a pub and ordered a "stout pale," which would have been a stronger pale ale, or a "brown stout," or "stout porter," more like the stouts of today. Generally, however, by around the turn of the 19th century, "stout" referred to a strong porter. Records from the Whitbread brewery in 1805 show that at that time they were producing a porter and three stouts- single, double, and triple stout. With the exception of the double stout, which had some amber malt added, the beers were made from precisely the same recipe, and the stronger ones simply used less water during the sparge, so that the resulting product was richer and more dense. By 1844 there were 5 different versions, all using precisely the same ingredient list, but varying in strength from around 4% to upwards of 9%. The styles then began to diverge during the second half of the 19th century: while porters started using patent malt, stouts began getting less patent malt and more brown malt, so that while stouts were being brewed to varying degrees of strength, they were trending more towards sweet than porters. This trend culminated with the advent of the "milk stout" in the 1900s: a stout brewed with lactose sugar. The fact that there were over 250 years of different stouts and porters to choose from when craft beer began its revival in the 1970s explains why there is so much overlap between the styles today.
Today, if you pick up a “stout” or “porter” with no other designation, you could get something that's actually closer to a black IPA, like Avery's New World Porter, or you could end up with a dry, easy-drinking brown porter like Holy City Pluff Mud. The best rule of thumb for trying stouts or porters is to find out what flavors you like, and then ask a Green's associate for reccomendations based on your own tastes. So, if regular stout was brewed for England and up to 9% ABV, where did Imperial Stout come from? Imperial Stout was brewed in London for export to Russia from at least 1796, when it is mentioned in the diary of the painter Joseph Farington. It was from Thrale's (that later became the Anchor Brewery of England) of Southwark. His diary notes that it was a favorite of the empress (then Catherine The Great) and was consumed in vast quantities by all her court. When the tarriff came along, it forbade all beer except porter, because it was believed at the time that the only good porter had to be made from the Thames river water. We don't exactly know when the terms "Russian" and "Imperial" finally collided, with the evidence suggesting it may have been as late as 1934. There is no conclusive evidence that "Imperial" directly referred to the court of Russia, though it stands to reason. "Imperial," however, meaning the strongest and best from a particular producer, was used as early as 1821, and did not always refer to export ales or porter specifically. "Russian Stout" was common throughout the 1800s, though some breweries produced both a "Russian Stout" and an "Imperial Stout." We DO know, however: it was not brewed "because regular beer would freeze" as some have suggested, (if it were cold enough for porter to freeze in transit, the ocean that they were sailing on would also be frozen), but was brewed to the tastes of Catherine the Great's court, and was quite popular in England as well by the late 1800s.