The first line is coastal and frontier provinces and cities.

The second line is a province and city between the first and third lines.

The third line includes the vast areas west of Beijing-Guangzhou line, east of Wushaoling in Gansu, south of Yanmenguan in Shanxi and north of Nanling in Guizhou, including Sichuan, Yunnan, Guizhou, Qinghai and Shaanxi provinces, most of Shanxi, Gansu and Ningxia Hui Autonomous Region, as well as western Henan, western Hubei, western Hunan, western Hebei, northern Guangxi and northern Guangdong.

Third-line construction:

In 1960s and 1970s, China's strategic rear area construction centered on strengthening national defense was an important part of national defense construction and national economic construction.

The third-line construction is an extremely large-scale industrial migration process in China's economic history, which took place against the background of Sino-Soviet hostility and the US attack on the southeast coast of China.

The implementation of the third-line construction has made great contributions to strengthening China's national defense strength, improving the layout of productive forces and industrialization in the central and western regions. However, due to the backward social economy in the third-tier areas, it is difficult for established enterprises to operate and develop for a long time.