The "proper law" of the contract is the main system of law applied to decide the validity of most aspects to the contract including its formation, validity, interpretation, and performance. This does not deny the power of the parties to agree that different aspects of the contract shall be governed by different systems of law. But, in the absence of such express terms, the court will not divide the "proper law" unless there are unusually compelling circumstances. And note the general rule of the lex fori which applies the provisions of the "proper law" as it is when the contract is to be performed and not as it was when the contract was made.