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I've come across a rule, that as far as I can tell, all domain registrars share: That you cannot register a domain that has two hyphens in positions 3 and 4 of the desired domain name. Apart from that, you can put as many hyphens in your domain as you like.

Is there an explanation for this rule?

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I suspect it's something to do with Punycode - the way of encoding Unicode characters into ASCII for domain names. However I don't understand the Punycode algorithm, so I can't see if allowing xx-- would somehow break the algorithm.

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+1 It gets into arcane stuff I don't follow, and will probably never have to, but I think you're in the right area here. Section 9.1.2 of this document specifically calls out the situation as @Cuy describes it(positions 3 & 4), though I can barely understand what it's talking about. According to a reference here this seems like it's a weird leftover encoding safety requirement that may or may not go away someday. –  Su' May 4 '12 at 9:31
    
Thank you for the answers, paulmorriss and Su'. I thought it would have to do with character encoding, but I wasn't yet familiar with Punycode. I'll look into that a little. Again, thanks! –  Cuy May 4 '12 at 9:40
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Wikipedia has further details. –  Oliver Salzburg May 7 '12 at 17:51
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