This article is from the Mongolia FAQ, by Oliver Corff with numerous contributions by others.
Astonishing as it may sound, Chinese has been the writing of choice
for important Mongolian documents during the 13th and 14th century.
Chinese characters (a virtually canonical set of some 500 characters)
were used according to their pronounciation. Some characters failed to
render the pronounciation and were prefixed (or affixed) with
modifiers, small Chinese characters indicating whether the main
consonant (or `initial') of the syllable had to be pronounced in a
velar manner of not. The most important document written with Chinese
characters is the Secret History of the Mongols. It was an achievement
of the late 19th and the early 20th century to decypher the text and
restore its original Mongolian shape. The problems linked to this work
are manyfold: One has to understand Early Mandarin (the name of the
specific form of Chinese used for this script) phonology, and one has
to understand words which appear only in this text but no other
source, not even the famous Hua Yi Yi Yu or Barbarian Glossaries,
Chinese dictionaries of the Middle Ages dealing with a number of
Central and North-East Asian languages. The most promiment scholars
contributing to the understanding of these texts were the Japanese K.
Shiratori, the German E. Haenisch, the Japanese Hattori, to name just
a few.
Using Chinese characters for writing Mongolian had the big advantage
that a message encoded in this system was obscure to a Chinese
messenger but perfectly transparent to a Mongolian listener. Despite
this advantage of privacy, the system ceased to be used in the early
14th century.
 
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