Retroactive refund or credit given to a buyer who has purchased a product or service. Fair and equitable rebates are used simply as incentives available to all customers. So-called deferred (or exclusive-patronage) rebates are used by large vendors of perishables and consumer durable goods. To receive such a rebate, the purchaser must agree to buy certain goods or services exclusively from a particular vendor for a fixed period of time. Rebating was a common pricing tactic in the 19th century, often used by large companies to undercut competition from smaller firms. The U.S. railroad industry practiced price discrimination by granting secret rebates to important customers; the rebates granted to Standard Oil Co. helped it acquire a monopoly over the oil industry. Governments occasionally sponsor rebates on previously paid taxes such as property, income, or sales taxes. Tax rebates can be used to stimulate the economy, to return excess revenue to taxpayers, or to encourage specific actions such as energy conservation.