In countries with such a system, <strong>Surtax</strong> on income tax is a mandatory local tax that you pay as an additional burden exclusively to the city or municipality where you have your registered residence (or habitual abode).
Surtax was not calculated as a percentage of the total gross salary, but was always calculated exclusively as a percentage of your income TAX amount. That is why it was colloquially called "tax on tax".
Surtax = (Income Tax Amount) × (Surtax Rate)The level of the surtax percentage is usually determined by <em>local city or municipal assemblies/councils</em>. They didn't even have to prescribe it! Large cities traditionally have higher rates, while smaller, prosperous places often abolish it or keep it at 0% to attract residents.