We've all heard those double or triple letters before ship names, right? Like the HMS Pinafore (a Gilbert and Sullivan opera), the RMS Titanic (the big, famous ship that sank), and the USS Enterprise (the starship manned by Captain Kirk). Plus there's SOS, which isn't a prefix but Morse Code that many mistakenly take to mean "save our souls" or "save our ships." Then there's plain old SS, an acronym for "steamship." "SS" was the first such ship prefix back in the 1800s when steamships took off as cutting-edge travel and shipping technology. And we mentioned "RMS." That's because the Titanic, in addition to ferrying 2,240 passengers, held between 6 to 9 million pieces of mail when it sank. That's because the Titanic, like all RMS vessels, was a delivery ship. "RMS" means "Royal Mail Ship."

Starting with the very first passenger-carrying steamship in 1807, the Clermont by American engineer Robert Fulton, steamship production and proliferation soared through the 19th century. By 1836, we saw the first transatlantic passenger steamship, the SS Great Western. Note the prefix "SS," which the Clermont didn't have. At this point, ships started getting prefixes based on roles and types, particularly steamships. Examples include designations we mentioned before, like "HMS" for His/Her Majesty's Ships, a prefix reserved for the British Royal Navy. Then there's "RMS," which also comes from the United Kingdom and means precisely what we said: A letter and package-carrying steamship vessel.