Using keywords in URLs is beneficial for best page SEO Optimization but is it better to use dashes or underscores to separate words?

This question has been asked many times and answer is quite simple, at least when it comes to optimizing for Google but I’m sure it can be applied for other two top Search Engines (Yahoo and MSN).

Most SEO Professionals will tell you that answer is “dashes” and here is why.

If you go to Google and search for any term in three different formats: first just the term, second separate words with dashes and third separate with underscores you will get totaly different results.

I’ve used “seo optimization” term for this test and here are the results:

seo optimization 8,070,000 Google results
seo-optimization 509,000 Google results
seo_optimization 478 Google results

From results you can see that Google treats the dash (-) as a ‘wildcard’ where the underscore (_) is treated like any other character and must have a exact hard match to bring it up in search results. In other words if you use underscore in your urls you will only benefit from url SEO if the person searches with “_” in their query.

So I would definitely recommend using dashes instead of underscores in your web pages URLs. There is probably a good reason why WordPress is automatically creating your posts and pages using dashes and the reason is better SEO.

What do you think?