When the government of the Republic of China drew a U‐shaped line around the islands on a map of the South China Sea in 1947, the decision‐makers were only referring to the islands within this line. Because the Republic of China held just 3 nautical miles of territorial waters, they could not possibly be thinking of including the sea areas within this U‐shaped line. However, with the development of the international Law of the Sea, especially the 1982 UN Convention on the Law of the Sea, heated discussions arose around the legal status of this U‐shaped line. It had been regarded as the islands’ attribution line, a historic waters line, a historical rights line, and a state boundary line. This article uses official archives to trace the original thinking of these decision‐makers and to infer the meaning of this U‐shaped line.