A customs duty is a charge assessed by a government on an imported item at its point of customs entry into the country, and paid for by the importer; the term is now used interchangeably with tariff. In terms of assessing duties there are two basic types: an ad valorem duty is assessed in proportion to the value of the imported item, whereas a "specific" duty is assessed on the basis of a measure other than value, such as the quantity of the product imported. In addition, a "compound" or "mixed" duty, which is a combination of an ad valorem and specific duty, is occasionally used in the harmonised Tariff Schedules of the United States (HTSUS). Special duties such as anti-dumping duties or countervailing duties may also be levied on imports to offset the unfair price advantage of an imported article that is sold below normal value or subsidised by an exporting country.