The classical or traditional Mongolian script, also known as the Hudum Mongol bichig, was the first writing system created specifically for the Mongolian language, and was the most widespread until the introduction of Cyrillic in 1946. It is traditionally written in vertical lines . Derived from the Old Uyghur alphabet, it is a true alphabet, with separate letters for consonants and vowels. It has been adapted for such languages as Oirat and Manchu. Alphabets based on this classical vertical script continue to be used in Mongolia and Inner Mongolia to write Mongolian, Xibe and, experimentally, Evenki.
Computer operating systems have been slow to adopt support for Mongolian script; almost all have incomplete support or other text rendering difficulties.
The Mongolian vertical script developed as an adaptation of the Old Uyghur alphabet for the Mongolian language. From the seventh and eighth to the fifteenth and sixteenth centuries, the Mongolian language separated into southern, eastern and western dialects. The principal documents from the period of the Middle Mongol language are: in the eastern dialect, the famous text The Secret History of the Mongols, monuments in the Square script, materials of the Chinese–Mongolian glossary of the fourteenth century and materials of the Mongolian language of the middle period in Chinese transcription, etc.; in the western dialect, materials of the Arab–Mongolian and Persian–Mongolian dictionaries, Mongolian texts in Arabic transcription, etc. The main features of the period are that the vowels ï and i had lost their phonemic significance, creating the i phoneme (in the Chakhar dialect, the Standard Mongolian in Inner Mongolia, these vowels are still distinct); inter-vocal consonants γ/g, b/w had disappeared and the preliminary process of the formation of Mongolian long vowels had begun; the initial h was preserved in many words; grammatical categories were partially absent, etc. The development over this period explains why the Mongolian script looks like a vertical Arabic script (in particular the presence of the dot system).