Citation, Documentation of Sources

Q. I am wondering why one needs to provide the URL for a journal or newspaper if one consults it online, but not the name of the library, say, if one consults it in print form? Typically everything about the articles is the same, and so the place where one found them should be irrelevant. Indeed, if I understand the logic, if one downloaded the PDF of a book, one would need to provide the URL, but if one made a PDF of a book and then read that, one wouldn’t have to. What am I missing?

A. The problem is that electronic editions of an article aren’t always the same. Writers or editors may tinker with them, adding updates and corrections. In contrast, a specific impression of a printed book or article will be the same as other physical copies of that impression. For now, the best way for a reader to know exactly which version of an electronic document was consulted (and to be able to find it) is to have the DOI or URL.