The Mongolian language historically has three writing systems that were used over the centuries.
Two writing systems based on simplified Chinese ideograms and Sinogram-typed alphabetic block (see Hangul), respectively, were used to write the Mongolic language of Khitan, also used to write the Tungusic Jurchen language in their modified forms. These two systems, called "Khitan/Jurchen big characters" and "Khitan/Jurchen small characters" fell into disuse when North China reverted to a homogenous Han Chinese culture.
Intermediate between these is the Mongolian script proper, in 12-13 centuries derived from the Uighur alphabet, descendant of Sogdian alphabet which came from Syriac alphabet. Perhaps its two most notable features are that it is a vertical script, and that it is the only such script that is written from left to right. (All other vertical writing systems are written right to left.) In fact, the Uighurs changed the orientation of their script from horizontal to vertical to emulate the Chinese writing system. The visual effect is that of Syriac rotated 90 degrees counterclockwise. This alphabet is reasonably accurate with respect to the representation of consonants, but fails to distinguish several vowels. The situation is somewhat comparable to the various dialects of English, which must represent 10 or more vowels with only 5 letters.
See also: Languages of China