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But you can write your categories in any way you please. Their choice of main categories won't suit every directory, and you should be creative and consider carefully the kind of sites you want to list. I like to categorize mine first according to the purpose of the site (business, fun, political, etc), then create subtopics according to subject areas.
One thing you will find is that most sites don't fit neatly into your slots anyway. No matter how carefully you think things out, something will always come along that demands a revision of your category structure.
100 Philosophy & psychology
300 Social sciences
500 Natural sciences & math arrow
600 Technology (Applied sciences)
700 The arts
800 Literature & rhetoric
900 Geography & history
Kids and Teens
The Open Directory is supposed to be based around the Dewey system used for libraries, I believe.
I can't remember where I read that. But taking a good look at Dmoz's About pages, I notice they don't mention Dewey at all, so I was wrong.
Unfortunately, this mostly leaves us with making it up as you go along. You could also sort domains alphabetically, by submission date, or by domain extension; but none of those methods will do enough to help users find what they are looking for.