You may have heard that it’s usually better to use a hyphen (-) than an underscore ( _ ) in your URLs for SEO purposes. This is true, Matt Cutts from Google talks about the subject here, but why?

The short answer is because the people who build the search engine indexes are programmers, the long answer is probably too complex for most humans to understand.

However you can try it for yourself by opening a simple text editor such as notepad and pasting the following strings into it.

double-click-this-word-string
double_click_this_word_string

When double-clicking the string with hyphens you can see that only one word at a time is highlighted. However if you double-click the string with underscores you’ll see that the entire string of words is highlighted. It’s treated as a single word.

As Matt writes in the post mentioned above. “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′.”

So if you want the file name component of your URL to be indexed as multiple keywords use a hyphen to separate the words, (eg. mydomain.com/word1-word2.html).

It is important to mention at this point that I’m NOT suggesting that you change your domain name to include hyphens, (word1-word2.com), this has very limited value in the SEO context and has a lot of drawbacks in other contexts. More about this coming in another post.