Articles 31 and 32 of the Vienna Convention, which set out the main rules of treaty interpretation, are commonly perceived as coming into play after the treaty enters into force. This chapter looks at the less well known but equally significant ex ante role of these provisions during the treaty formation process in providing tools for the negotiator to use to safeguard interests or to bridge differences to arrive at an agreement. These can take the form of annexes and footnotes inserted in the treaty text, side agreements, or statements that record a shared understanding on the interpretation of certain provisions. The legal efficacy of annexes, footnotes, and side agreements is fairly clear. It is, however, less certain for statements that record a shared understanding on interpretation because of the diverse forms they may take and the circumstances in which they are made.