Duress has been defined as a “threat of harm that is made to force a person to do something against his will or judgment; esp., an illegitimate threat made by one person to force a manifestation of another person`s apparent consent to a transaction without real will.  An example is Barton v Armstrong  in a person who has been threatened with death if he does not sign the treaty. An innocent party wishing to impose a contract of coercion on the person only has to prove that the threat was made and that it was one of the reasons for entering the contract; the burden of proof then rests with the other party to prove that the threat had no effect on the performance of the contract by the party. There may also be constraints on goods and sometimes “economic constraints.” An exception arises when advertising makes a unilateral promise, such as offering a reward, as decided in the famous case of Carlill v Carbolic Smoke Ball Co, in 19th century England. The company, a pharmaceutical manufacturer, proposed a smokeball that, if it sniffed “three times a day for two weeks,” would prevent users from catching the “flu.” If the smokeball does not prevent “the flu, the company promised that it would pay $100 to the user, adding that they deposited “$1000 in the Alliance bank to show our sincerity in the file.” When Ms. Carlill complained about the money, the company argued that the complaint should not be considered a serious and legally binding offer; instead, it was a “simple mess”; However, the Court of Appeal found that Carbolic had made a serious offer to a reasonable man and found that the reward was a contractual undertaking. Sometimes the ability of individuals or artificial persons to enforce or enforce contracts is limited. For example, very young children should not be seen as good deals they have done assuming they do not have the maturity to understand what they are doing; Employees or managers may be prevented from entering into contracts for their company because they have acted in an ultra vires manner (beyond their power). Another example could be people who are unable to act mentally, either because of a disability or through drunk driving.  Written contracts may consist of a standard form agreement or a letter confirming the agreement.