Article 32 of the Vienna Convention on the Law of Treaties 1969 deals with supplementary means of interpretation of treaties
· "Recourse may be had to supplementary means of interpretation, including the preparatory work of the treaty and the circumstances of its conclusion, in order to confirm the meaning resulting from the application of article 31, or to determine the meaning and interpretation according to article 31: (a) leaves the meaning ambiguous or obscure; or (b) leads to a result which is manifestly absurd or unreasonable.”
This preparatory work of a treaty is known as the travaux préparatoire. The documents included in travaux préparatoires include successive drafts of the treaty, conference records, explanatory statement by an expert consultant had a codification conference, uncontested interpretive statements by the chairman of the drafting committee and International Law Commission commentaries.
commentary on the meaning of article 32 an example of its usage may be found in:
Aust, Anthoony. Modern Treaty Law and Practice. Cambridge; New York: Cambridge University Press, 2013
Villiger, Mark Eugen. Commentary on the 1969 Vienna Convention on the Law of Treaties. Leiden: Martinus Nijhoff Publishers, 2009.
published compilations of travaux préparatoires, as well as websites pulling together the underlying materials:
The United Nations maintains a site Repertory of Practice of United Nations Organs
Yale Law School Goldman Library maintains a web page devoted to collected travaux préparatoires, here
Examples of compiled travaux préparatoires include: