The regions of Taiwan are based on historical administrative divisions. However, most of the definitions are not precise.
Division into two regions
Eastern and Western Taiwan: the Central Mountain Range separates Taiwan into east and west.
Eastern Taiwan: Yilan, Hualien and Taitung.
Western Taiwan: other divisions from Taipei to Pingtung.
Northern and Southern Taiwan: Zhuoshui River, the longest river of Taiwan, flows through about the middle of the island.
Northern Taiwan: Taipei, New Taipei, Keelung, Taoyuan, Hsinchu (City/County), Miaoli, Taichung, Changhua, and Nantou.
Southern Taiwan: Yunlin, Chiayi (City/County), Tainan, Kaohsiung, and Pingtung.
Division into four regions
The most widely used definition is from the Council for Economic Planning and Development (經濟建設委員會), Executive Yuan. This division into four regions (tetrachotomy) scheme corresponds to the prefectures under Qing dynasty rule.
Division into five regions
The scheme of division into five regions (pentachotomy) is a fusion of the tetrachotomy and hexachotomy schemes. Although no specific names are given in each division, it is the most commonly used scheme among the highest divisions of the central government. This scheme is used by the Joint Service Centers (JSC, 區域聯合服務中心) under Executive Yuan (行政院) and the jurisdiction of High Court Branches (高等法院分院) under Judicial Yuan (司法院).
Division into six regions
The division into six regions (hexachotomy) scheme corresponds to the prefectures under Japanese rule. This scheme was used for national electoral districts in the legislative elections in 1972, 1975, 1980, 1983, and 1986. The discussion of this scheme became popular after the elections of five new municipalities in 2010.