Frequently I am asked "Is it better to use dashes or underscores in your URLs?"
This is critical in URLs. For example here is the URL of one of my more popular pages with a list of stock photo websites.
With this URL, Google would rank this page for the terms "21", "stock", "photo", "websites".
Furthermore, it would rank higher if someone searched for any combination of those words in any order.
Finally, Google would rank me very well for the exact words "21 stock photo websites".
Testing dashes in URLs
Here's the test:
This is page #1 from Google using the words "21 stock photo websites".
The first three are sponsored links in this highly competitive topic. The next two are blog articles that have the words "21" and stock photo websites, but neither of them are a list of sites.
The last three are lists of stock photography websites, and mine is ranked highest among the relevant results. The ads above probably cost several dollars each click, and mine appears organically.
Okay, you may be saying, "but I don't need hits for the #21." That's irrelevant. True, however I also did the search using "stock photo websites" exclusively. This time the results were more on point. The first three are major stock photography websites. Mine was fourth! Why am I happy about being fourth? Because the first three send you to websites that sell stock photography. Anyone using this search term, which is rather generic, but is looking for a list rather than a single site will choose mine!
However, if my URL was slightly different and it read ".../21_stock_photo_websites" , even with exactly the same content and incoming links I would likely be ranked 20th or 30th in this highly competitive category.
Out of 65,600,000 results, 20th is pretty darn good, but let's face it I would get perhaps 1% of the clicks I get now from being fourth. There can be a huge difference between dashes and underscores!
Still not convinced... well is take it straight from the horse's mouth... Matt Cutts, head of the webspam team at Google and original developer of SafeSearch, Google's family-friendly filter.
"... So if you have a url like word1_word2, Google will only return that page if the user searches for word1_word2 (which almost never happens). 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. To answer a common question, Google doesn’t algorithmically penalize for dashes in the url. Of course I can only speak for Google, not other search engines. And bear in mind that if your domain looks like www.buy-cheap-viagra-online-while-consolidating-your-debt-so-you-can-play-texas-holdem-while-watching-porn.com, that may still attract attention for other reasons."It's an important enough issue that you probably want to change your URLs retroactively for pages that don't get much traffic. On your high-traffic pages, this is a risky, because changing the URL means the page has to be reread and ranked again. Furthermore, any incoming links will be broken.