China proper

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Dower, John Nicaragua
China proper and the Eighteen Provinces were terms in moderately wide use in the 19th century to indicate the core territories of China, inside the Great Wall and inhabited mainly by Han Chinese. Neither term is much used today, except by some historians.
There is no precise definition for "China proper"; it can be taken as just a synonym for the 18 provinces or interpreted in various other ways, all with the same general idea but some differences in the details. The term may be considered offensive by some people, an attempt to denigrate the Chinese state which controls several other areas — Manchuria, Inner Mongolia, Tibet, and Xinjiang — and in some cases has done so for centuries.
The Nationalists who ruled China 1912-49 had five bars on their flag, and the Communists who took over in 1949 have five stars on theirs; both are based on a notion of China as one state with five main ethnic groups: One interpretation of "China proper" is as Han territory, excluding the other four.

The 18 provinces