IANA blocked registration of most of them under gTLDs (.COM, .NET, etc.) in 1993, on the thought they may be useful to support extensibility of the DNS down the track. Country-code operators have typically followed that lead as good practice.

As they were never really used for that purpose, slowly but surely they have been released into circulation. The applications by registry operators to lift the one- and two-letter domain restrictions are at http://www.icann.org/en/resources/registries/rsep

You can have zero character domain names. Briefly trying out a few of the country TLDs, the one for Anguilla works: http://ai. the trialing dot is not needed but helps tell your browser not to try to expand it further.

Many TLDs do not resolve, though many others resolve to the TLD's registrar's site.

A TLD can have an A record, so in theory you could have a "0 character" domain. You could also use it for email, so a@co would be a valid email address if co. had an A or MX record associated with it.

I guess it's all about the money. The fact the amazon could afford it and I nor you couldn't explains it better, I guess.

I'm not sure about any others except Google uses g.co for shortening their own URLs.

Edit: And Twitter uses t.co

I have always wondered about x.com and how they got that domain. Does anyone around here know?

Twitter uses t.co