The parties may exclude the application of these Principles or derogate from or vary the effect of any of their provisions, except as otherwise provided in the Principles.
1. The non-mandatory character of the Principles
The rules laid down in the Principles are in general of a non-mandatory character, i.e. the parties may in each individual case either simply exclude their application in whole or in part or modify their content so as to adapt them to the specific needs of the kind of transaction involved (see the Model Clause in the footnote to the second paragraph of the Preamble).
2. Exclusion or modification may be express or implied
The exclusion or modification of the Principles by the parties may be either express or implied. There is an implied exclusion or modification when the parties expressly agree on contract terms which are inconsistent with provisions of the Principles and it is in this context irrelevant whether the terms in question have been negotiated individually or form part of standard terms incorporated by the parties in their contract.
If the parties expressly agree to the application of some only of the Chapters of the Principles (e.g. “As far as the performance and non-performance of this contract is concerned, the UNIDROIT Principles shall apply”), it is presumed that the Chapters concerned will be applied together with the general provisions of Chapter 1.
3. Mandatory provisions to be found in the Principles
A few provisions of the Principles are of a mandatory character, i.e. their importance in the system of the Principles is such that parties should not be permitted to exclude or to derogate from them as they wish. It is true that given the particular nature of the Principles the non-observance of this precept may have no consequences. On the other hand, it should be noted that the provisions in question reflect principles and standards of behaviour which are of a mandatory character under most domestic laws also.
Those provisions of the Principles which are mandatory are normally expressly indicated as such. This is the case with Article 1.7 on good faith and fair dealing, with the provisions of Chapter 3 on substantive validity, except in so far as they relate or apply to mistake and to initial impossibility (see Article 3.1.4), with Article 5.1.7(2) on price determination, with Article 7.4.13(2) on agreed payment for non-performance and Article 10.3(2) on limitation periods. Exceptionally, the mandatory character of a provision is only implicit and follows from the content and purpose of the provision itself (see, e.g., Articles 1.8 and 7.1.6).