A tariff is a list or schedule of taxes, while a duty is the tax imposed on a specific item. However, the terms duty and tariff have come to be used interchangeably. In international trade, these taxes must be paid to a government on selected imported or sometimes exported goods. The Harmonized Tariff Schedules of the United States (HTSUS) lists the items on which the United States levies duties. Tariffs may be protective of domestic producers (keeping domestic prices higher than world prices) or serve as revenue generators for the government. Tariffs are considered transparent trade barriers in contrast to several nontariff barriers. The Uruguay Round Agreement on Agriculture requires conversion of nontariff barriers to bound tariffs.