A territorial sea baseline is the starting line from which the width of a coastal country's territorial sea, and certain other maritime zones under its jurisdiction, are measured. The water areas located within the line and linked to the land are considered internal waters. The water areas outside of the line then, are first territorial waters, and after that jurisdictional waters, including the Exclusive Economic Zone and the Continental Shelf.

The Japanese government's decision to "purchase" the Diaoyu Islands sparked a determined response from China. On Sept. 10, 2012, the Chinese government announced the baselines of the territorial waters of the Diaoyu Islands and their affiliated islets, as well as the names and coordinates of 17 base points, in accordance with the Law of the People's Republic of China on Its Territorial Seas and Adjacent Zones. China's action is also consistent with the spirit of the United Nations Convention on the Law of the Sea.

The Chinese government has reiterated that the Diaoyu Islands are an inalienable part of China's territory and China will make no concessions on issues concerning its sovereignty and territorial integrity. Meanwhile, the Chinese government announced that it will conduct a normalized surveillance and monitoring of the Diaoyu Islands and its affiliated islets.

领海基线是测算沿海国的领海宽度和其他管辖海域宽度的起算线，基线里面向陆地一侧的水域就是内水，向海的一侧首先是领海，后面还会有专属经济区大陆架各种管辖海域。

中国政府对日本政府“购买”钓鱼岛的决定做出了坚决的回应。根据《中华人民共和国领海及毗连区法》，中国政府于九月十日宣布了中国钓鱼岛及其附属岛屿的领海基线和17个领海基点的名称和地理坐标。中国的相关做法符合《联合国海洋法公约》。

中国政府重申，钓鱼岛及其附属岛屿是中国领土不可分割的一部分，在涉及主权和领土完整的问题上，中国决不退让。同时，中国政府还宣布将对钓鱼岛及其附属岛屿开展常态化监视监测。