In terms of conciseness and compactness, Classical Chinese rarely uses words composed of two Chinese characters; nearly all words are of one syllable only. This stands directly in contrast with modern Northern Chinese varieties including Mandarin, in which two-syllable, three-syllable, and four-syllable words are extremely common, whilst although two-syllable words are also quite common within modern Southern Chinese varieties, they are still more archaic in that they use more one-syllable words than Northern Chinese varieties. This phenomenon exists, in part, because polysyllabic words evolved in Chinese to disambiguate homophones that result from sound changes. This is similar to such phenomena in English as the "pen–pin" merger of many dialects in the American south and the "caught-cot" merger of most dialects of American English: because the words "pin" and "pen", as well as "caught" and "cot", sound alike in such dialects of English, a certain degree of confusion can occur unless one adds qualifiers like "ink pen" and "stick pin." Similarly, Chinese has acquired many polysyllabic words in order to disambiguate monosyllabic words that sounded different in earlier forms of Chinese but identical in one region or another during later periods. Because Classical Chinese is based on the literary examples of ancient Chinese literature, it has almost none of the two-syllable words present in modern Chinese varieties.