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Hu Huanyong, the man behind the concept, never defined the boundary as a straight line. Rather, a chain of city network of which Chinese culture divided east-west demographically due to availability of fertile plain that support population growth.

In your mentioned region, Sichuan, lies Chengdu plain which border the mountain range. Making the demographic line also meander along from Beichuang to Songpan to Chengdu. There's another meander along Hanzong further north, and Yunnan in the south.