'Phags-pa script
The 36 initials are a traditional classification of initial onsets used in Chinese books dating back to the Tang dynasty (618–907), but by the Yuan dynasty they represented an idealized phonetic system that did not accurately reflect the Old Mandarin language that the 'Phags-pa script was designed to represent. The discrepancy between the theoretical and actual phonology of Yuan dynasty Chinese is indicated by certain peculiarities in the use of 'Phags-pa letters to represent the 36 initials in "Menggu Ziyun":