On the design of URLs: users/alice/book/chapter3#section1

"To the right are identifiers for objects in the world of hypertext. ... To the left are the bits about networking. In the marriage of networking and hypertext that is the web, that hash sign is where they meet."
"If you learn anything about the design of the web, let it be this: the design merges networking and hypertext, and they meet at the hash sign. If you get that, then you basically get it." - @timbl