Permalinks: Why are hypens(-) used over underscores(_) to replace spaces(and other unwanted characters)?

Question

In some large websystems I have come across lately, friendly permalinks, e.g. part of a HTML path that is based on a (often user-specified) string rather than a numerical number, spaces(and other unwanted/disallowed characters that would otherwise need to be url-escaped) are replaced by hyphens (-), and not by underscores (_).

An example: in the URL http://example.com/blog/this-is-my-first-post, this-is-my-first-post is a friendly permalink. Using underscores, this would be http://example.com/blog/this_is_my_first_post

Is this only a personal preference, or is there a technical reason to use hyphens over underscores?

Hypothetical possibilities I thought of:

• Maybe it matters for Search Engine Optimalization?
• Maybe it is actually important for how HTML paths are interpreted?
• Maybe there is a historical reason?

What I do know:

• Hyphens are treated as word-breaks in most (if not all?) computer systems/programs, e.g. use ctrl+left/ctrl+right to move in a sentence_that_uses_underscores vs a sentence-that-uses-hyphens.
• In normal text that a user enters (e.g. names for objects or blogposts), usage of actual hyphens is higher than underscores.

Could someone shine some light on this?

Answer 1

Google has spoken:

Consider using punctuation in your URLs. The URL http://www.example.com/green-dress.html is much more useful to us than http://www.example.com/greendress.html. We recommend that you use hyphens (-) instead of underscores (_) in your URLs.

https://support.google.com/webmasters/answer/76329?hl=en