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I never use the hyphen, but as always, I have seen pages that use hyphens, underscores, and all that stuff, that rank in the top 5.
(If it does, I have some serious renaming to do)
There's nothing more annoying then trying to explain to someone that there's a hyphen in your domain name. I don't know how many times I've been asked "How do I spell that?" :) It probably doesn't do anything bad/good with SEO, but there's a bit of user confusion. I'd run the words together, if that option is open.
It seems pretty standard to use _ in file names, but then, I've also seen people who use file names where multiple words are designated by capitalizing the first letter of each word - RunThemAllTogether.zip for example.
However, I have a couple hyphenated domains that are used strictly for promotional use for affiliate programs. They are fantastic in this regard. I don't care if anyone remembers the domain just so long as they can find it in the SEs.
Using the Google cache as a starting point of logic, I see that keywords that stand alone are highlighted, while when part of a compound run-on word, they are not. I have also seen, over and over, with different search engines, that there will be different results with the stand-alone word in the page titles, and headings, as opposed to compound words.
Example: search on handcrafted whatever will yield a different result than hand crafted whatever. I've got one page from a site using both forms, done to check this out, and it will yield a different ranking for the use of the the same phrase using the singular and compound versions.
This is, I believe, a stemming issue. Now, if compound words make a difference in page titles and page text, will there be the same interpretation with either domain names or filenames, following along using this logic?
I believe that underscores or hyphens will have the same effect as using stand-alone keywords as opposed to runonwords in other placs, but I stand ready to be corrected on this.
A relevant issue is whether or not there is a boost from using keywords in domain names and file names. This could have one answer today and a different answer in a month, but my reasoning is that it's best to stay with the course of highest possibility and assume the positive, that it will give a boost - even though the run-on is easier to type, more user friendly, and takes less characters up, since there's a limit to how long the file path should be. I believe 35 characters is the optimum maximum length.
A good point now is which search engines are attributing added value to keywords in filenames and domain names at this time, and which aren't. And also, for which ones is it necessary to use stand alone words.
At this point in time, I'm inclined to go with hyphens and underscores.
BIG ditto on that. Never, ever, ever have a hyphenated domain name unless it's a throw-away or you're covering all the bases and it's backup for the run-on version you own as well. <added> The negatives are not related to SEO, but to other marketing problems, as others have pointed out. I have several Top3 listings of hyphenated domains in the major engines.</added>
But don't miss the post about word1word2 search kws. JohnQ is becoming more and more likely to search on it.
I must disagree -- it all depends.
The statement omits an important consideration, because Google is extremely hot on recognizable keywords in domain names (at least they were last month -- who knows next month?). To be recognizable for Google, it has to be hyphenated unless it's a stand-alone domain word (with periods on either side).
To answer the question, then, requires additional questions.
1) Is there the possibility of getting an important keyword or two into the domain name by using a hyphen? (Often the answer is, "No, they're all taken already," which solves your problem right away.)
Another "No" might be the nature of the words themselves. Searchengineworld and webmasterworld are two examples of names that should be run together. No one in their right mind would search for terms like this on the web, because they'd get too many hits. They'd have to zero in on something more specific. But there are other examples where you can get useful "KEY" keywords into the domain name, and for these examples, you have to hyphenate.
2) If yes, then are you more interested in Google ranking than the inconvenience of saying "hyphen" on the telephone?
I just started a site with two of my most important keywords in the domain: www.XXX-on-YYYYY.org
I got "I feel lucky" on Google on its first index (with about 4 inbound links), and that's out of 44,000 hits for the two words, XXX YYY together in the Google search box (no quotes in the Google box).
Others just underneath me also have one of the keywords in the domain, but not both.