Ex fida bona
Ex fida bona is a Latin expression and a principle of Roman law.
The term means that a judge is to make a judgement based on "good business norms" and that the contractual parties are to keep their words, thus making it possible for both to trust each other. An agreement should be according to the branch norms unless otherwise specifically mentioned.
This was a condition to permanent trade relations during the rise of Rome. It was during the 2nd century BC that the Roman praetors started using this principle, as commerce grew in the Mediterranean.
