To be bound by a contract, a person must have the legal ability to enter into a contract called contractual capacity. A person who, because of his or her age or mental disability, is unable to understand what he or she is doing when he or she signs a contract, may not be able to enter into a contract. For example, a person who is protected by law because of a mental defect is in no position to agree. Any contract signed by this person is not valid. Skills differ in their principles of contractual freedom. In common law laws such as England and the United States, a high degree of freedom is the norm. In American law, for example, in the case of Hurley v. Eddingfield, the physician was allowed to refuse treatment to a patient, despite the lack of other medical care available and the subsequent death of the patient.  This runs counter to civil law, which generally applies certain cross-cutting principles to contract disputes, as in the French civil code. Other legal systems, such as Islamic law, socialist legal systems and customary law, have their own variations.
Contracts are generally verbal or written, but written contracts have generally been favoured in common law legal systems;  In 1677, England passed the Fraud Act, which influenced similar fraud laws in the United States and other countries such as Australia.  As a general rule, the single code of commerce, as adopted in the United States, requires a written contract for the sale of material products over $500, and real estate contracts must be written. If the contract is not prescribed by law, an oral contract is valid and therefore legally binding.  Meanwhile, the United Kingdom has replaced the original Fraud Act, but written contracts are still required for various circumstances such as the country (by property law in 1925). The effect of these conditions is a matter of interpretation of the parties` agreement. In any case, it will be a question of fact. However, in general, the marking of the agreement as “treaty-compliant” means that it cannot itself be a contract and therefore cannot be applied.