I found something about 'unpreceded' adjectives but could not quite understand what it means. Unpreceded by what?

No preceding article. As in "Black skirts" instead of "the black skirt" or "a black skirt". When there is no preceding article, strong inflection is used: http://en.wikipedia.org/wiki/German_declension#Strong_inflection.5B6.5D.5B7.5D

I believe "Two black skirts" falls into the category of "no preceding article".
I'm not a native speaker though, so feel free to correct me if I'm wrong.