Context: A treaty is an official, express written agreement that states use to legally bind themselves. A treaty is the official document which expresses that agreement in words; and it is also the objective outcome of a ceremonial occasion which acknowledges the parties and their defined relationships.

Question: How are the agreements in a treaty expressed?
Answer: in words

Question: How are states that enter into a treaty bound?
Answer: legally

Question: What is an official document which expresses an agreement between two states?
Answer: A treaty

Question: What is an objective outcome of a ceremonial occasion which acknowledges the defined relationships of its parties?
Answer: A treaty

Question: Who is responsible for the legally-bound obligations of the parties to a treaty?
Answer: themselves