A flat tax is just what it sounds like—every person in the country is taxed at the exact same rate, in Cruz’s case 10% for individuals and 16% for business. This is a stark difference from America’s current progressive tax code, where households with higher incomes are taxed at a higher marginal rate, topping out at 39.6%. Supporters of a flat tax claim that it is more fair and makes it easier for people to know what they owe the government, rather than having a complicated set of deductions to figure out which tax bracket you end up in.

Cruz’s plan is nothing new to American politics. It has been a favorite of tax reformers for decades. One of the earliest modern proponents of the flat tax was Democrat Jerry Brown, currently the governor of California and a long-time figure in the Democratic Party. Brown ran for president in 1992, with a flat tax of 13% for both personal income taxes and businesses as part of his platform, according to a 1995 article from The Heartland Institute.

After that, though, the flat tax became a favorite plank of conservative Republican, especially those whose bona fides were based mainly on being a “fiscal conservative.” Former House Majority Leader Dick Armey, a veteran of the political wars of the 1990s, was, and is, a staunch proponent of the idea. Many Americans first heard about the flat tax during the 1996 Republican primary elections, when candidate and businessman Steve Forbes based his entire campaign around the notion.

Since then, myriad candidates have advocated for the flat tax or some variation of it. This year, Mike Huckabee and Rick Santorum are on board, along with Cruz. In 2012, Herman Cain became famous (or infamous, as the case may be) for his “9-9-9” plan, which included a 9% flat tax for individuals.

(A quick aside — don’t confuse the flat tax with the FairTax. The FairTax is a branded plan backed by some fiscal conservatives that calls for the removal of all income taxes and the implementation of a consumption tax, essentially a national sales tax on all purchased goods and services. Many say this plan is regressive, as the middle class would end up paying more as a percentage of income.)

Russia has had a flat tax of 13% since 2001. And it was instituted by Vladimir Putin (makes you wonder if Ted Cruz knows he has this in common with the Russian president, who has been frequently demonized by the Republicans during this campaign.) Nine U.S. states have a flat tax (including Tennessee and New Hampshire, which only tax interest and dividend income). The highest flat rate is in North Carolina at 5.75%, and the lowest is Pennsylvania at 3.03%. Most of these states have some personal deduction—that is, income that isn’t taxable, so if you have income of less than that figure you pay no taxes. Pennsylvania, though, has a true flat tax. If your income for the year is $100, you owe $3.03.

Reforming the tax code is a massive undertaking, and even if a Republican made it to the White House with a Republican congress, there is no guarantee they could get it done. But with Cruz, a fairly serious candidate, making it a key part of his platform, expect it to be a big part of the election debates over the next year.