A poll tax, also known as a head tax or capitation, is a tax of a uniform, fixed amount applied to an individual in accordance with the census (as opposed to a percentage of income, or any proxy for ability-to-pay). Head taxes were important sources of revenue for many governments from ancient times until the 19th century. They have also been used in the past, notably in the United States, to disenfranchise minority voters. In the United Kingdom, poll taxes were levied by the governments of John of Gaunt in the 14th century, Charles II in the 17th and Margaret Thatcher in the 20th century.