Agreement

According to Sir John Salmond, an agreement is also a source of law . He said “ That an agreement operates a source of rights is a fact too familiar to require illustration.” If X and Y enter into an agreement which is a lawful one , the courts of law recognize that agreement and enforce the same on X and Y . The same is the case if A and B enter into a agreement with a lawful purpose . However, such agreement bind only the parries to the parties not to the others. 

Law is a rule of conduct and generality is the test of law. There is no generality in an agreement between two parties. An agreement is recognized so long as t exists, and when it is dissolved , it has no farther effect. Agreement plays an important part in international law . There may be an agreement among number of State to follow particular procedure with regard to a certain matter . 

The States entering into the agreement are bound by that . If the agreement is continued for a long time by a large number of States , it acquires the force of a custom and thus custom is born as a source of International law. That's why Keeton Says , custom in a source of international law , but agreement never it so; agreement is merely is a source custom and then if a number of other agreement exist compelling uniformity of conduct of the States who are parties to them. 

The process of municipal law is the same. A number of agreements concluded in the same way and enforcing similar courses of conduct on the parties to them may cause a custom to grow. But it is the custom and not the agreement which is always the source of law.