In legal usage, quid pro quo indicates that an item or a favour has been traded in return for something of value, usually when the propriety or equity of the transaction is in question. For example, under the common law, a binding contract must involve consideration: that is, the exchange of something of value for something of economic value. If the exchange appears excessively one sided, the court may question whether a quid pro quo exists and the contract may be voidable.