This article is a study of gogae-related back morphemes of Korean toponyms, which are characteristic of geographic typology. In order to investigate the original forms and their development and regional distribution, I analyzed toponyms that appear in representative geography books of each historic period: Old Korean (before tenth century), Middle Korean (tenth to sixteenth century), and Modern Korean (seventeenth century to present). Gogae 고개 and jae 재 are two original forms of back morphemes of the gogae-related toponyms in vernacular Korean. Jiui 知衣appearing in the Samguk sagi jiriji (Geographical Appendix to the Historical Records of the Three Kingdoms), compiled in 1145, can be regarded as the transcription of jae into a Chinese character. Other back morphemes in Sino-Korean characters (hanzi 漢字) include ryeong 嶺, hyeon 峴, jam 岑, jeom 岾, and chi 峙. In Modern Korean, ryeong, hyeon, and chi are the most widely used back morphemes in South Korea, whereas only ryeong is used in North Korea.