China has been criticized by some countries for making "ambiguous" claims on the islands, islets, reefs and waters in the South China Sea. For example, it has been criticized for "failing to honor" the United Nations Convention on the Law of the Sea despite being a signatory to it, as well as for "violating" other international laws on the sea.

A few international observers also accuse China of deliberately obscuring its territorial claims in the South China Sea by using terms not found in the UNCLOS, such as "adjacent waters" and "relevant waters". And some countries keep demanding that China "clarify" its nine-dash line map.

The fact is that, if these countries do not change their mindset and attitude, the nine-dash line will continue to be vague for them irrespective of how clearly China defines it.

China has an unequivocal and consistent territorial claim on the islands and other land features in the South China Sea. As a matter of fact, it has unequivocally stated its claim in three official documents: the 1947 Location Map of the South China Sea Islands released by the Kuomingtang government in Nanjing, the 1958 Declaration of the Government of New China on the Territorial Sea and the 1992 Law on Territorial Sea and Contiguous Zone. These documents state that the Dongsha Islands, Xisha Islands, Zhongsha Islands, Nansha Islands and other islands are part of the sovereign territory of China.

Some countries view China's maritime claim in the South China Sea as ambiguous because of certain historical reasons. The first reason is that the UNCLOS does not properly address the issue of historic rights. Despite the reference to historic title in Articles 15 and 298(1)(a), the provision on historic bays in Article 15(6), and the recognition of traditional fishing rights in Article 51, it does not have any provision for the definition of historic rights or their specific connotation and denotation.

The second is that no consistent understanding has been reached in international law on historic rights. For example, Yehuda Z. Blum, an Israeli professor of law and diplomat, has observed: The term "historic rights" denotes the possession by a state, over certain land or maritime areas, of rights that would not normally accrue to it under the general rules of international law, such rights having been acquired by that state through a process of historical consolidation ... Historic rights are a product of a lengthy process comprising a long series of acts, omissions and patterns of behavior which, in their entirety, and through their cumulative effect, bring such rights into being and consolidate them into rights valid in international law.

Besides, a state acquires historic rights through effective exercise of these rights (long series of acts, omissions and patterns of behavior) by one or more states, a practice followed by relevant states. The concept of historic rights is almost equivalent to that of historic water.

In this vein, Leo Bouchez, a renowned international law professor, says the concept of "historic rights" has evolved from the concept of "historic water" and "historic bays". The development from "historic bays" to "historic water" and from "historic title" to "historic rights" indicates the evolution of legal concepts with the development of state practice, and that such concepts have not been finalized.

From the point of view of China, one of the world's oldest civilizations, the South China Sea is part of the traditional Asian order and, hence, it would be inappropriate to comprehend the nine-dash line by relying solely on the Westphalian nation-state system.

As Keyuan Zou, Harris professor of International Law at the University of Central Lancashire in the UK, has observed, the South China Sea nine-dash line map was officially released by the Chinese Kuomingtang government half a century before the UNCLOS, and one decade before the 1958 Four Geneva Conventions on the Law of the Sea. Thus, China's historic rights within the nine-dash line cannot be ignored. The nine-dash line drawn by the Chinese government in 1947, at approximately the median position between China's South China Sea islands and reefs and the coastlines of bordering states, reflects the scope of China's claims. The consistency of the claims has been maintained by China after 1949, and the claims have been recognized or acquiesced to by bordering states over a long period of time. Therefore, the nine-dash line has probative force and weight under international law.

The so-called ambiguity in China's nine-dash line map and its claim on the waters within that line mainly stems from the imperfection of the UNCLOS. To some extent, international law on historic rights is defective in theory and doctrine and lacks a unified standard.

China has been striving to clarify its claim in the South China Sea. But the joint efforts of the international community are also needed to complement and improve the UNCLOS by agreeing to a new international convention or protocol in order to clarify the understanding of historic rights.

The author is deputy director of Maritime Security Institute at East China University of Political Science and Law in Shanghai. The views expressed are personal. His most recent book is Legal Interpretation of China's South China Sea Map: An Inclusive Approach to Ocean Public Order.