The Three Kingdoms of Korea, with Goguryeo and Buyeo in blue.
It is unknown except for a small number of words, which mostly suggest that it was similar but not identical to the language of Silla, and influenced by the Tungusic languages. Striking similarities between Baekje and Goguryeo can also be found, which is consistent with the legends that describe Baekje being founded by the sons of Goguryeo's founder. The Goguryeo names for government posts are mostly similar to those of Baekje and Silla.
Some historical linguists have attempted to link Buyeo, Goguryeo, and Baekje with Old Japanese. Others argue that the connections to Japanese may be due to earlier languages of southern Korea, such as perhaps Gaya, and that Goguryeo–Baekje was closer to Silla and Korean. Words of Goguryeo origin can be found in Middle Korean (early 10th to late 14th century).
- Beckwith, C. I. (2004). Koguryo, the language of Japan's continental relatives: an introduction to the historical-comparative study of the Japanese Koguryoic languages with a preliminary description of Archaic northeastern Middle Chinese. Brill's Japanese studies library, v. 21. Boston: Brill. ISBN 90-04-13949-4
- Beckwith (2006). "Methodological Observations on Some Recent Studies of the Early Ethnolinguistic History of Korea and Vicinity." Altai Hakpo 2006, 16: 199-234.
- Beckwith (2006). "The Ethnolinguistic History of the Early Korean Peninsula Region: Japanese-Koguryoic and Other Languages in the Koguryo, Paekche, and Silla Kingdoms." Journal of Inner and East Asian Studies, 2006, Vol. 2-2: 34-64.
- Beckwith (2007): Koguryo, the Language of Japan's Continental Relatives: An Introduction to the Historical-Comparative Study of the Japanese-Koguryoic Languages, with a Preliminary Description of Archaic Northeastern Middle Chinese. Brill Academic Publishers, 2004. ISBN 90-04-13949-4. Second edition, 2007. ISBN 90-04-16025-6
- List of Korea-related topics
- History of the Korean language
- Buyeo languages
- Old Korean
- Japonic languages
- Classification of Japonic