Written Cantonese is the most complete written form of Chinese after that for Mandarin Chinese and Classical Chinese. Written Chinese was originally developed for Classical Chinese, and was the main literary language of China until the 19th century. Written vernacular Chinese first appeared in the 17th century, and a written form of Mandarin became standard throughout China in the early 20th century. While the Mandarin form can in principle be read and spoken word for word in other Chinese varieties, its intelligibility to non-Mandarin speakers is poor to incomprehensible because of differences in idioms, grammar and usage. Modern Cantonese speakers have therefore developed new characters for words that do not exist and have retained others that have been lost in standard Chinese.