The Mongolian vertical script was developed as an adaption of the Uyghur script to write the Mongolian language. It was introduced by the Uyghur scribe Tatar-Tonga, who had been captured by the Mongols during a war against the Naimans around 1204. There were no substantive changes to the Uyghur form for the first few centuries, so that, for example, initial yodh stood for both [dʒ] and [j], while medial tsadi stood for both [dʒ] and [tʃ], and there was no letter for in initial position. Mongolian sources often distinguish the early forms by using the term Uyghurjin script. Western sources tend to use this term as a synonym for all variations of the Mongolian script.