Since the pronunciation of all modern varieties of Chinese is different from Old Chinese or other forms of historical Chinese (such as Middle Chinese), characters that once rhymed in poetry may not rhyme any longer, or vice versa. Poetry and other rhyme-based writing thus becomes less coherent than the original reading must have been. However, some modern Chinese varieties have certain phonological characteristics that are closer to the older pronunciations than others, as shown by the preservation of certain rhyme structures. Some believe Classical Chinese literature, especially poetry, sounds better when read in certain varieties that are believed to be closer to older pronunciations, such as Cantonese or Southern Min, because the rhyming is often lost due to sound shifts in Mandarin.