The double slash, though a programming convention at the time, turned out to not be really necessary, Mr. Berners-Lee explained. Look at all the paper and trees, he said, that could have been saved if people had not had to write or type out those slashes on paper over the years — not to mention the human labor and time spent typing those two keystrokes countless millions of times in browser address boxes. (Today’s browsers, of course, automatically fill in the “http://” preamble when a user types a Web address.)
This is nostalgic. My thesis required me to research the history of Internet, and his name pops out a lot. Funny though, I never really wondered the use of the double slash. It's like, in Christianity, God exists. No explanations given.