South Korea's first postal code was introduced on July 1, 1970. Individual codes were provided for each post office distribution area, using an nnn-nn numeric format (e.g., 120-01 represented Susaek-dong, Seodaemun-gu, Seoul). Unlike current postal coding provided by administrative districts, the 1970 system was implemented based upon a post office's railroad line; this often led to inconsistencies in post codes relative to their administrative areas.