'Phags-pa script
The rhyming conventions of the new verse were codified in a rime dictionary called the "Zhongyuan Yinyun" (1324). A radical departure from the rime table tradition that had evolved over the previous centuries, this dictionary contains a wealth of information on the phonology of Old Mandarin. Further sources are the 'Phags-pa script based on the Tibetan alphabet, which was used to write several of the languages of the Mongol empire, including Chinese and the "Menggu Ziyun", a rime dictionary based on 'Phags-pa. The rime books differ in some details, but overall show many of the features characteristic of modern Mandarin dialects, such as the reduction and disappearance of final plosives and the reorganization of the Middle Chinese tones.