Question

When you are building URLs that should be legible for users and search engines and you do it automatically from the content, what’s the best way to represent blank spaces? Hyphens (this is what StackOverflow uses)? Underscores? Any other? Does any of those make a different for SEO?

Answer 1

Both are valid URL characters and both have their pros and cons.

Pro dash

• Google recommends dashes, and here is what Matt Cutts from Google has to say about
  Dashes vs. underscores.

  If you have a url like word1-word2,
  that page can be returned for the
  searches word1, word2, and even “word1
  word2″.
  That’s why I would always choose
  dashes instead of underscores.

• Dashes seem to be what major blogs do:
  The Huffington Post,
  TechCrunch,
  Engadget, …
• Dashes seem to be what major CMS do.
  Not sure about that one anymore, can anyone comment?
• As mentioned by Kazar, underscores can clash with the underlining of links.
• I find underscores awkward to type.
• Rene Saarsoo pointed out that dashes take less space than underscores in proportional fonts.
• Ionut G. Stan mentioned that underscores are not allowed in hostnames. If you strive for consistency you should opt for dashes.

Pro underscore

• Dashes are not allowed in
  
  ISO9660 file systems.
  This can be a problem if your content is also shipped on DVD or CD (e.g help files or
  eLearning content).
• In some languages (e.g. German) dashes can be word characters and are not generally considered word separators.