In countries with such a system, the Surtax 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).
The surtax was not calculated as a percentage of the total gross salary, but was always calculated as a percentage exclusively on your INCOME TAX amount. That is why it was colloquially called a "tax on a tax".
Surtax = (Income Tax Amount) × (Surtax Rate)The surtax percentage rate is usually determined by local city or municipal assemblies/councils. 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 population.