Pacta sunt servanda is a brocard, a basic principle of civil law

Civil law (legal system)

Civil law is a legal system inspired by Roman law and whose primary feature is that laws are codified into collections, as compared to common law systems that gives great precedential weight to common law on the principle that it is unfair to treat similar facts differently on different...

 and of international law

International law

Public international law concerns the structure and conduct of sovereign states; analogous entities, such as the Holy See; and intergovernmental organizations. To a lesser degree, international law also may affect multinational corporations and individuals, an impact increasingly evolving beyond...

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In its most common sense, the principle refers to private contract

Contract

A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...

s, stressing that contained clause

Clause

In grammar, a clause is the smallest grammatical unit that can express a complete proposition. In some languages it may be a pair or group of words that consists of a subject and a predicate, although in other languages in certain clauses the subject may not appear explicitly as a noun phrase,...

s are law

Law

Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

 between the parties, and implies that nonfulfilment of respective obligations is a breach of the pact. The general principle of correct behavior in commercial practice — including the assumption of good faith

Good faith

In philosophy, the concept of Good faith—Latin bona fides “good faith”, bona fide “in good faith”—denotes sincere, honest intention or belief, regardless of the outcome of an action; the opposed concepts are bad faith, mala fides and perfidy...

— is a requirement for the efficacy of the whole system, so the eventual disorder is sometimes punished by the law of some systems even without any direct penalty incurred by any of the parties.

With reference to international agreements, "every treaty

Treaty

A treaty is an express agreement under international law entered into by actors in international law, namely sovereign states and international organizations. A treaty may also be known as an agreement, protocol, covenant, convention or exchange of letters, among other terms...

 in force is binding upon the parties to it and must be performed by them in good faith

Good faith

In philosophy, the concept of Good faith—Latin bona fides “good faith”, bona fide “in good faith”—denotes sincere, honest intention or belief, regardless of the outcome of an action; the opposed concepts are bad faith, mala fides and perfidy...

." Pacta sunt servanda is based on good faith.