It can be applied to individual taxes or to a tax system as a whole; a year, multi-year, or lifetime. Proportional taxes maintain equal tax incidence regardless of the ability-to-pay and do not shift the incidence disproportionately to those with a higher or lower economic well-being.
Flat taxes, implemented as well as proposed, usually exempt from taxation household income below a statutorily determined level that is a function of the type and size of the household. As a result, such a flat marginal rate is consistent with a progressive average tax rate. A progressive tax is a tax imposed so that the tax rate increases as the amount subject to taxation increases.[4][5][6] The opposite of a progressive tax is a regressive tax, where the tax rate decreases as the amount subject to taxation increases.