Why HTTP:// contain two slashes and file:/// three

Question

I want to ask why http:// contains two slashes is it just standard to make it URL or any logically meaning? and why file:/// contains three slashes?

file:///C:/a.html

Answer 1

The authority component of a URI has to be preceded by //:

The authority component is preceded by a double slash ("//") […]

This is also why not all URIs contain the double slash: because not all URIs have an authority component (e.g., URIs using the mailto scheme, the xmpp scheme, etc.).

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If you wonder why the double slash instead of something else (or nothing) was chosen for (HTTP) URIs, see Tim Berners-Lee’s FAQ Why the //, #, etc? → What is the history of the //?

tl;dr: He copied the filename syntax which Apollo used.

By the way, he regrets that choice:

I have to say that now I regret that the syntax is so clumsy. I would like http://www.example.com/foo/bar/baz to be just written http:com/example/foo/bar/baz where the client would figure out that www.example.com existed and was the server to contact. But it is too late now.

Answer 2

As mentioned in this superuser post:

The complete syntax is file://host/path.

If the host is localhost, it can be omitted, resulting in file:///path.

In other words, referring to files in your computer is just like referring to files in localhost.