“三线”这一概念出自毛主席关于三线建设的战略构想。在这个构想中,他把全国划分为前线、中间地带和三类地区,分别简称一线、二线和三线。这些地区位于我国腹地,离海岸线最近700公里以上,距西面国土边界上千公里,加之四面分别有青藏高原、云贵高原、太行山、大别山、贺兰山、 The concept of “three lines” is derived from Chairman Mao’s strategic concept of the third-line construction. In this idea, he divided the country into front lines, middle zones and three types of areas, namely the first, second and third lines respectively. These areas are located in the hinterland of our country, more than 700 kilometers away from the coastline and thousands of kilometers away from the borders of the western territories. In addition, there are four areas, namely the Qinghai-Tibet Plateau, the Yunnan-Guizhou Plateau, the Taihang Mountains, the Dabie Mountains, the Helan Mountains,