The document discusses the use of the comparative structure "as...as" to make comparisons between two items or aspects. It can be used with adjectives, nouns, and adverbs to express equivalent qualities or manners. When used with adjectives and adverbs, "as...as" indicates equality, while "not as...as" shows inequality. With nouns, it also compares amounts that are equal or unequal. The structure follows the pattern of "be + as + noun/adjective/adverb + as".