The boundary between the Old and Middle periods is traditionally identified with the establishment of Goryeo in 918, but some scholars have argued for the time of the Mongol invasions of Korea (mid-13th century).

It is difficult to extract linguistic information from texts of the Early period, which are written with Chinese characters (called Hanja in Korean).

Until the late 19th century, most formal writing in Korea, including government documents, scholarship and much literature, was written in Classical Chinese.

[9][10] Many of the gugyeol characters were abbreviated, and some of them are identical in form and value to symbols in the Japanese katakana syllabary, though the historical relationship between the two is not yet clear.

[2] The Hunminjeongeum ('The Correct/Proper Sounds for the Instruction of the People') and later texts describe the phonology and morphology of the language with great detail and precision.

[16] The tensed stops pp, tt, cc and kk are distinct phonemes in modern Korean, but in LMK they were allophones of consonant clusters.

[18] The tensed fricative hh only occurred in a single verb root, hhye- 'to pull', and has disappeared in Modern Korean.

[23] Late Middle Korean had a limited and skewed set of initial clusters: sp-, st-, sk-, pt-, pth-, ps-, pc-, pst- and psk-.

[43] As a result, over half the modern Korean lexicon consists of Sino-Korean words, though they account for only about a tenth of basic vocabulary.

[47] This system became so entrenched that 15th-century efforts to reform it to more closely match the Chinese pronunciation of the time were abandoned.

[49] Some of these denoted items of imported culture, but it was also common to introduce Sino-Korean words that directly competed with native vocabulary.