There is a legal distinction between the obligation of an executory contract, that is, one not yet performed or carried out on one side at least, and an executed contract, that is, one which has been fully carried out on both sides; and it has been held that the constitutional guaranty extends to contracts fully executed as well as to those which are in whole or in any part still executory. This conclusion was reached in a case in which a state attempted to impair the effect of a conveyance of land made by it to an individual, and it was held that as a conveyance was in this sense a contract, the title acquired thereby could not be impaired or affected by the state action (Fletcher v. Peck). This decision was made, however, before the adoption of Amendment XIV which prohibits any state from depriving any person of his property without due process of law. Under that amendment any attempt on the part of the state by statute to impair a property right would be invalid, and since the adoption of that amendment the decision that a state cannot impair the rights acquired under an executed contract is probably of little significance, for such rights would now be protected as property rights.

There is also a legal distinction between express and implied contracts, an express contract being one which is definitely entered into between parties intending to contract and bind themselves with reference to each other, while an implied contract is nothing more than an obligation arising by law from the acts of the parties without any expressed intention to assume such obligation. An implied contract in the proper sense of the term is one the obligation of which a party is presumed to have assented to by reason of his conduct and his relations to the other party, although such assent is not indicated by any specific words or acts, and the constitutional guaranty applies to such implied contracts as fully and effectively as to expressed contracts. The term "implied contract" is sometimes, though inaccurately, used to cover any legal obligation, such as, for instance, the obligation to pay damages for a wrong done, although such wrong is not a violation of any duty specifically assumed but only of a duty generally imposed by law. The obligations arising from implied contracts when the term is used in the sense last above indicated are not obligations which are within the guaranty of the constitutional provision as to impairing the obligation of contracts (Louisiana v. Mayor of New Orleans).