Looking for capitals such that every combination of three letters in it, is in some other capital. e.g. if "Lisbon" were an answer, there would have to be at least one other city with "LIS", "ISB", "SBO", and "BON". Trigrams going over spaces or dashes don't count, they have to be in the same word (apostrophes are okay though). Also, I went by the "Capitals of the World" quiz, but I know some capitals have alternate spellings and some countries have alternate capitals, and if they make a difference in another one's qualification for the quiz, it'll be a bonus.