The question is how does Google (or Bing, Yahoo, etc.) see a hyphen or an underscore and how can one utilize this knowledge to achieve maximum keyword effectiveness. In general when querying a search engine, a hyphen is the same as a space. An underscore, however, is treated as a character.

On all the major search engines, querying a long tail keyword phrase in quotes specifies only mentions of those keywords in that order will be returned on a search engine results page. On Bing placing hyphens between keywords returns a similar result.

This can most easily be verified by comparing the number of results returned for a given long tail keyword phrases on search engine results pages (SERPs). While one can accomplish this, of course, by visiting each search engine, there are a few tools to facilitate comparison of SERPs on different search engine.

Search Engine Dance (Google, Yahoo, Bing): http://j.mp/6c1EsE [Would have posted addition tools had the comment CMS permitted.]

For example, query the words "maximum", "keyword" and "effectiveness" as a single long tail keyword phrase in all the possible variants.

maximum keyword effectiveness
maximum-keyword-effectiveness
maximum_keyword_effectiveness
"maximum keyword effectiveness" (with quotation marks)

Obviously, the number of results returned differ. Given that John Q. Public will most likely only query a long tail keyword phrase without hyphens, underscores, or quotation marks, using hyphens in a domain name is the solution that will render the results which will most closely render similar results (with the exception of Bing). However, please note, that if you've achieve sufficient search engine optimization (SEO) to place well for a given long tail keyword phrase without the use of a hyphenated domain name, doing so will only increase the occurrence of favorable placement on the SERPs.

Thanks Jen,

You caught me a bit off-guard with this as the site isn't really officially up and running yet, however, my preference has always been hyphens, so www.domain.com/i-would-do-this

I'm don't think it should make any difference, but it is a good question!

Hopefully we can find an expert to clarify.

Cheers, Pete

I think that the best way to write it is www.mydomain.com/this-is-my-url.html

But I'm thinking about what's the real difference between the other solutions...

I think that Google prefers dashes, but I can't remember where I got that.

http://www.mattcutts.com/blog/dashes-vs-underscores/

Matt Cutts said dashes - but that was a long time ago.