The following question appeared on an SEO Forum blog in September 2005:
I had read that separating keywords in domain names was purely a matter of choice, or does this only relate to file names - webdesign.html -v- web-design.html being the same?

The response provided to this question:
There are two different issues involved with this question. The first concerns the characters that are allowed in a domain name and/or a URL, the second concerns how special characters are treated by the search engines.

For example, Nominet rules limit the characters allowed for a .uk TLD (Top-Level Domain) to:
• the twenty-six unaccented Roman letters (i.e. a-z inclusive);
• the ten western digits (i.e. 0-9 inclusive);
• and hyphens.

So the only possible separator character to use in a .uk domain is the hyphen e.g. keyword1-keyword2.co.uk.

There are fewer restrictions on which characters can be used in a URL (i.e. in the directory and file name section of a web address). Safe characters are the 26 letters, 10 western digits and $ _ - . + ! * ‘ ( )

All other characters are either reserved - have a specific meaning in a URL (e.g. / ? : @) - or unsafe (e.g. # ~ [ ] ^). Reserved characters must be and unsafe characters should be encoded in their appropriate US ASCII notation. e.g. a space is encoded as %20, # as %23 etc.

So, in theory, you could equally well use an underscore as a hyphen in a file name as a separator between two keywords as in: example.com/keyword1_keyword2.html.

However, Google decided in its early days that it would index the underscore as a recognisable character and the hyphen as a word separator.

There is a small advantage in having keywords in your URL that are discernable by search engines and the most reliable way to achieve this is to use the hyphen, the only separator which is acceptable throughout the URL, including in the domain name.

domain names, keywords, url

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