Porter is a dark-coloured style of beer. The history and development of stout and porter are intertwined.[1] The name was first used in the 18th century from its popularity with the street and river porters of London. It is generally brewed with dark malts. The name "stout" for a dark beer is believed to have come about because a strong porter may be called "Extra Porter" or "Double Porter" or "Stout Porter". The term "Stout Porter" would later be shortened to just "Stout". For example, Guinness Extra Stout was originally called "Extra Superior Porter" and was only given the name Extra Stout in 1840.[2]

From wikipedia