[Sidenote: The two Sources of International Law.]
§ 16. As the basis of the Law of Nations is the common consent of the member States of the Family of Nations, it is evident that there must exist, and can only exist, as many sources of International Law as there are facts through which such common consent can possibly come into existence. Of such facts there are only two. A State, just as an individual, may give its consent either directly by an express declaration or tacitly by conduct which it would not follow in case it did not consent. The sources of International Law are therefore twofold--namely: (1) _express_ consent, which is given when States conclude a treaty stipulating certain rules for the future international conduct of the parties; (2) _tacit_ consent, which is given through States having adopted the custom of submitting to certain rules of international conduct. Treaties and custom are, therefore, exclusively the sources of the Law of Nations.
[Sidenote: Custom in Contradistinction to Usage.]
§ 17. Custom is the older and the original source of International Law in particular as well as of law in general. Custom must not be confounded with usage. In everyday life and language both terms are used synonymously, but in the language of the jurist they have two distinctly different meanings. Jurists speak of a custom, when a clear and continuous habit of doing certain actions has grown up under the ægis of the conviction that these actions are legally necessary or legally right. On the other hand, jurists speak of a usage, when a habit of doing certain actions has grown up without there being the conviction of their legal character. Thus the term "custom" is in juristic language a narrower conception than the term "usage," as a given course of conduct may be usual without being customary. Certain conduct of States concerning their international relations may therefore be usual without being the outcome of customary International Law.
As usages have a tendency to become custom, the question presents itself, at what time a usage turns into a custom. This question is one of fact, not of theory. All that theory can point out is this: Wherever and as soon as a frequently adopted international conduct of States is considered legally necessary or legally right, the rule which may be abstracted from such conduct, is a rule of customary International Law.