美国统一商法法典UCC（Uniform Commercial Code）以及普通法(Common Law)都适用于合同相关事宜，其中普通法适用大部分的合同。普通法既包括法院在个案审判中形成的判例法，也包括立法机关为法典化或促进普通法而进行的行为（如Statute of Frauds)。UCC第一及第二章适用于货物买卖合同。普通法有时也适用货物买卖合同，然而当其与UCC冲突时，优先适用UCC规则。普通法及UCC所管辖 的交易范围有细微差距。

1.合同法原则 A.基本定义 I.合同定义

a.      普通法下的合同

DEFINITION: Under common law, a contract is a legally enforceable agreement. In most instances, contract formation requires a bargain in which there is a manifestation of mutual assent to the exchange and also consideration. A valid contract is formed when there is an offer, an acceptance, and consideration.

b.      UCC规则下的合同

DEFINITION: Under the UCC, a contract is a total legal obligation that results from the parties’ agreement. In most instances, contract formation requires a bargain in which there is a manifestation of mutual assent to the exchange and also consideration. A valid contract is formed when there is an offer, an acceptance, and consideration.

合同种类

a.      双务合同(Bilateral Contract) 基于一个行为或承诺而成立

DEFINITION: Under common law, a bilateral contract is a contract in which mutual promises are given as the agreed exchange for each other.

i.双务合同示例

Anne承诺如果Bill同意浇灌Anne花园,其将支付Bill1000元，Anne承诺就浇灌行为向Bm支付1000元，Bill承诺为1000元进行浇灌。一旦Bill同意接受给付金额 及上述工作，合同即告成立。

b.      单方合同(Unilateral        Contract) 基于一个行为而成立

DEFINITION: Under common law, a unilateral contract is a contract in which a promise is given in exchange for actual performance by the other party. A unilateral contract can only be formed upon completion of performance; a mere promise to perform cannot bind the offeror.

i.       单方合同示例

示例一

Anne让Bill浇灌自己的花园，并且明确只有Bill完成才付款1000元，此时只有当 Bill切实完成工作，该合同才告成立。

示例二

Anne发布悬赏广告：谁能找到她的狗赏金100元。如若此时Bill致电承诺一定找到， Anne与Bill之间并无合同关系，即使Bill已经为此付出了巨大努力，但始终未找到。 只有当Bill真的找到了失踪犬只，Anne才须支付承诺款项。

ii.      单方合同v.双务合同

单方合同与双务合同有时难以明确区分。因此，现代法院一般都推定合同为双务合 同，除非要约中明确告知合同的单方性（即，只有被要约人的行为才构成承诺）。

c.      准合同 (Implied-in-Law Contract)

DEFINITION: Under common law, a quasi-contract is a legal substitute for a contract to be used when a court wishes to impose an obligation on a party to avoid unjust enrichment. In other words, if a party uses a technicality of contract law to avoid paying for a benefit actually received, II court may still force the party to pay some amount. A court may use a quasi-contract if: ⑴ a benefit was conferred to another, (2) there was a reasonable expectation of payment for the benefit conferred,⑶ the recipient of the benefit knew or had reason to know of expectation of payment, and (4) the recipient of the benefit will be unjustly enriched if he does not pay for the benefit.

i.准合同构成要件

只有当下述要件全部满足时，法院才会适用准合同规则：

1 )利益已经给付的事实；

2)      一方对于已给付之利益须支付对价拥有合理的期待；

3)      利益收受方明知或应知该等期待的存在；和

4)      如果利益收受方不支付该等对价，将导致其不当得利。

示例：

甲知晓邻居乙需要粉刷房屋。一天，甲通过粗放窗户发现粉刷工的卡车停在了甲的 房前。粉刷工开始对甲的房屋进行粉刷，甲在目睹粉刷全过程期间未曾提出任何异 议。在此示例中，甲与乙之间并不具有直接的合同关系，但法院为防止不当得利而 将判定甲与乙之间存在准合同关系。

d.      事实推定合同(Implied-in-Fact Contract)

DEFINITION: Under common law, an implied-in-fact contract is an agreement presumably intended by the parties to the agreement, but not reduced to express language.

i.       普通法项下事实推定合同

事实推定合同的成立并非基于合同双方的语言，而是双方的行为表现了其合意。该 行为能够体现双方对于合同成立的共同意愿。事实推定合同有时也被应用在不当得 利的情形下。

ii.      UCC项下事实推定合同

双方的行为显示合同已经订立、而无书面合同。此时事实推定合同的条款细节以双 方的书面记录或双方同意之事项为准。

iii.     事实推定合同示例

甲每日都在地铁报亭购买报纸，甲会习惯性地在报亭窗台处留下一元用以支付。一 日甲为赶地铁忘记放置一元于报亭。从甲的行为中可知，他一直都是按照正常价格 支付报纸费用，故甲与报亭之间构成事实推定合同，甲基于该事实推定合同而欠报 亭报款一元。

3.合同有效性

a.      无效合同(Void Contract)

DEFINITION: Under common law, a void contract is an agreement that is completely

without any legal effect from the beginning.

i.无效合同示例

雇凶合同无效。相似的，在立法禁止赌博的州，打赌也是无效的合同。

b.      可撤销合同(Voidable      Contract)

DEFINITION: Under common law, a voidable contract is an agreement that one or both parties can elect to avoid or ratify, but is not automatically without effect.

i.可撤销合同示例

合同一方主体不适格，合同为可撒销。如果一方主体为儿童，则大多数情况下，该 方（而非合同对方）有权撤销合同。

c.      不可履行合同(Unenforceable  Contract)

DEFINITION: Under common law, an unenforceable contract is an agreement that may be valid by formation，but is otherwise unenforceable due to an applicable defense.

抗辩事由包括：主体资格不适格(Incapacity)、胁迫及不当影 响(Duress and Undue Influence),过失及重大误解(Mistake and Misunderstanding)，其骗及不实陈述（Fraud and Misrepresentation)，显失公平(Unconscionability) 和 Statute of Frauds。

i.不可履行合同

有些合同本身合法，但因违反了 Statute of Frauds而变为不可履行合同。例如根据 Statue of Frauds,不动产买卖合同必须为书面合同且经双方签字方有效。如果上述二 条件未能满足，该不动产买卖合同在绝大多数情形下属于不可履行合同。

4.其他相关概念

自.UCC规则所述之货物

DEFINITION: Under the UCC, goods are all things that are movable at the time of identification to a contract for sale. Goods do not include regular cash or currency, but goods do include money being treated as a commodity. Goods also include the unborn young of animals and growing crops to be severed from land. |UCC § 2-105(1)|.

i. UCC规则所述之货物的说明

UCC规则所述之货物包括即将完成之产品、定制产品、尚未出生的牲畜、生长中的 农作物以及其他可辨别的实物。该等货物不包含信息、作为支付价款的货币、投资 证券、境外货币交易、可诉讼而获得之财产、服务及不动产。

a.      UCC规则所述之商户(Merchant)

DEFINITION: Under the UCC, a merchant is a person who regularly deals in the type of goods involved in the transaction, or who, through his occupation, has specialized knowledge of the business practice involved. |UCC § 2-104(1)1.