What is the reason that file URLs start with three slashes: “file:///” etc?

Question

I was wondering why file URLs have 3 slashes in front.

For example, if I were to read a pdf file using chrome, the URL would be file:///D:/Desktop/Book.pdf.

This seems totally unnecessary. Since http uses only two slashes, common sense would tell me that a file URL should look like this: file://D:/Desktop/Book.pdf.

Why do file URLs have three slashes?

Answer 1

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

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

See RFC 1738 – Uniform Resource Locators (URL):

A file URL takes the form:

file://<host>/<path>

[…]

As a special case, <host> can be the string "localhost" or the empty string; this is interpreted as 'the machine from which the URL is being interpreted'.

Answer 2

Dennis has explained the 3rd slash, needed to separate the host from the path, but the other two are much more interesting...

It turns out they were a useless and somewhat arbitrary addition to the URL syntax. Tim Berners-Lee, inventor of the World Wide Web and author of many of its standards (including the RFC that Dennis linked to), lamented his usage of the 'double slash' in an interview back in 2009.

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.

http://bits.blogs.nytimes.com/2009/10/12/the-webs-inventor-regrets-one-small-thing/

So, save for a minor (and uncharacteristic) lapse in foresight some 18 years ago, your file URL could just have easily been file:/D:/Desktop/Book.pdf, rather than file:///D:/Desktop/Book.pdf.

There is, to answer your question, no good reason why URLs have 3 slashes.