根据当事人双方是否存在对待给付义务，可以将合同分为双无合同和单务合同。其中，当事人双方互负对待给付义务的合同为双务合同，仅有一方负给付义务的合同为单务合同。具体如下：

 

Bilateral Contract 

双务合同

A bilateral contract is an agreement between two parties in which each side agrees to fulfill their side of the bargain. Typically, bilateral contracts involve an equal obligation or consideration from the offeror and the offeree, although this need not always be the case.

双务合同是双方之间的一种协议，在此类协议中，双方均同意履行其在交易中的义务。一般而言，双务合同涉及要约人和受要约人的同等义务或代价，尽管情况不一定总是如此。

 

In more complex situations, such as multinational trade negotiations, a bilateral contract can be a so-called "side deal." That is, both parties are involved in the general negotiations but may also see the need for a separate contract relevant only to their shared interests.

在更复杂的情况下，比如跨国贸易谈判，双务合同可以是所谓的“附带协议”。即双方均参与了一般性谈判，但也可能认为有必要订立一项仅涉及其共同利益的单独合同。

 

How a Bilateral Contract Works

双务合同如何运作

The bilateral contract is the most common kind of binding agreement. Each party is both an obligor (a person who is bound to another) to its own promise, and an obligee (a person to whom another is obligated or bound) on the other party's promise. A contract is signed so that the agreement is clear and legally enforceable.

双务合同是最常见的一种具有约束力的协议。各方既是自身承诺的义务人(对另一方有约束力的人)，也是另一方承诺的权利人(对另一方有义务或约束的人)。签订合同是为了使协议明确并可依法执行。

 

Any sales agreement is an example of a bilateral contract. A car buyer may agree to pay the seller a certain amount of money in exchange for the title to the car. The seller agrees to deliver the car title in exchange for the specified sale amount. If either party fails to complete one end of the bargain, a breach of contract has occurred.

销售协议都是双务合同。如汽车买家可能同意支付给卖家一定金额的钱，以换取汽车的所有权。卖方同意交付汽车所有权以换取指定的销售金额。如果任何一方未能完成交易的一端，违约已经发生。

 

In that sense, virtually all of our daily routine transactions are bilateral contracts, sometimes with a signed agreement and often without one.

从这个意义上说，我们几乎所有的日常交易都是双务合同，有时会有已签署的协议，但往往没有。

 

Business contracts are almost always bilateral. Businesses provide a product or service in exchange for financial compensation, so most businesses are constantly entering into bilateral contracts with customers or suppliers. An employment agreement, in which a company promises to pay an applicant a certain rate for completing specified tasks, is also a bilateral contract.

商业合同几乎总是双务合同。企业提供产品或服务以换取财务补偿，因此大多数企业不断与客户或供应商签订双务合同。雇佣协议也是一种双务合同，在雇佣协议中，一家公司承诺按一定的费率向申请人支付完成特定任务的费用。

 

Unilateral Contract

单务合同

 

A unilateral contract is a contract agreement in which an offeror promises to pay after the occurrence of a specified act. In general, unilateral contracts are most often used when an offeror has an open request in which they are willing to pay for a specified act.

单务合同是要约人承诺在特定行为发生后进行支付的合同协议。一般而言，单务合同最常用于要约人有一个公开请求，其中他们愿意为某一特定行为付款。

 

An example of a unilateral contract is an insurance policy contract, which is usually partially unilateral. In a unilateral contract, the offeror is the only party with a contractual obligation. A unilateral contract is distinguished from the more common bilateral contract.

单务合同的一个例子是保单合同，通常部分是单务合同。在单务合同中，要约人是承担合同义务的唯一一方。单务合同有别于更为常见的双务合同。

 

Understanding Unilateral Contracts

理解单务合同

Unilateral contracts specify an obligation from the offeror. In a unilateral contract, the offer or promises to pay for specified acts that can be open requests, random, or optional for other parties involved.

单务合同规定了要约人的义务。在单务合同中，要约人或承诺为指定的行为付费，这些行为对其他相关方来说可以是公开的请求，随机的，或可选择的。

 

Unilateral contracts are considered enforceable by contract law. However, legal issues typically do not arise until the offeree claims to be eligible for remuneration tied to acts or occurrences.

根据合同法，单务合同被视为可强制执行。然而，在受要约人声称有资格获得与行为或事件相关的报酬之前，通常不会出现法律问题。

 

As such, legal contestation generally involves cases where the offering party refuses to pay the offered sum. The determination of contract breach would then depend on whether or not the terms of the contract were clear and if it can be proven that the offeree is eligible for payment of specified acts based on the contract’s provisions.

因此，法律争议通常涉及要约方拒绝支付要约金额的情况。因此，违约的确定将取决于合同的条款是否明确，以及能否证明受要约人有资格根据合同的规定获得特定行为的付款。

 

Bilateral vs. Unilateral Contracts

双务合同 vs. 单务合同

 

As noted, a bilateral contract by definition has reciprocal obligations. That makes it distinct from a unilateral contract. In a unilateral contract, one party is obligated to fulfill its obligation only if and when the other party completes a specified task. A unilateral contract typically involves the first party issuing a payment only on completion of the second party's task.

如上所述，双务合同顾名思义具有互惠义务,使其有别于单务合同。在单务合同中，一方只有在另一方完成特定任务时才有义务履行其义务。单务合同通常涉及一方仅在完成另一方的任务时付款。

 

In legal terms, that second party in a unilateral contract is not obliged to actually perform the task, and may not be found in breach of contract for not doing so. If it were a bilateral contract, both parties would have a legal obligation.

从法律上讲，单务合同的另一方没有义务实际履行任务，而且可能不会因为履行任务而被认定为违反合同。如果是双务合同，则双方均有法律义务。

 

An example of a unilateral contract might be a contest to find a buried treasure to win $1 million. No one is obligated to hunt for the treasure, but if someone finds it, the contest creator is obliged to pay $1 million to that person. If the nature of a contract is disputed, a court will judge the merits of the claim against the content of the contract, determining if one or both parties maintain an obligation or concession.

单务合同的一个例子，寻找埋藏的财宝以赢得100万美元。没有人有义务去寻找宝藏，但如果有人找到了，竞赛的发起者有义务支付100万美元给这个人。如果对合同的性质有争议，法院将根据合同的内容判断索偿的是非曲直，确定一方或双方是否履行义务或作出让步。

 

英文来源：

https://www.investopedia.com/terms/b/bilateral-contract.asp

https://www.investopedia.com/terms/b/bilateral-contract.asp