Since Incoterms provide a set of terms for use in different trades and regions it is impossible always to set forth the obligations of the parties with precision. To some extent it is therefore necessary to refer to the custom of the port or of the particular trade or to the practices which the parties themselves may have established in their previous dealings (cf. article 9 of the 1980 United Nations Convention on Contracts for the International Sale of Goods). It is of course desirable that sellers and buyers keep themselves duly informed of such customs when they negotiate their contract and that, whenever uncertainty arises, they clarify their legal position by appropriate clauses in their contract of sale. Such special provisions in the individual contract would supersede or vary anything that is set forth as a rule of interpretation in the various Incoterms.