Legality is not a key element of a valid negotiation contract. The key elements are offer, acceptance, and consideration.

Duress is the legal doctrine that states that a contract is void if it is entered into under duress or coercion.

Mistake is the legal doctrine that states that a contract is void if one of the parties is mistaken about a material fact.

Unconscionability is the legal doctrine that states that a contract is void if it is so one-sided that it is unfair to one of the parties.

Illegality is the legal doctrine that states that a contract is void if it is entered into for an illegal purpose.

Minority is the legal doctrine that states that a contract is void if it is entered into by a minor.

Insanity is the legal doctrine that states that a contract is void if it is entered into by a person who is mentally incompetent.

Intoxication is the legal doctrine that states that a contract is void if it is entered into by a person who is intoxicated.

Drug Influence is the legal doctrine that states that a contract is void if it is entered into by a person who is under the influence of drugs.

Hypnosis is the legal doctrine that states that a contract is void if it is entered into by a person who is under the influence of hypnosis.

Undue Influence is the legal doctrine that states that a contract is void if it is entered into by a person who is under the influence of undue influence.

Misrepresentation is the legal doctrine that states that a contract is void if it is entered into by a person who is under the influence of misrepresentation.

Duress is the legal doctrine that states that a contract is void if it is entered into by a person who is under the influence of duress.

Coercion is the legal doctrine that states that a contract is void if it is entered into by a person who is under the influence of coercion.

Undue Influence is the legal doctrine that states that a contract is void if it is entered into by a person who is under the influence of undue influence.