<p>Last-minute income tax filer Jen Makowski works on her taxes at the main post office, April 15, 2004, in Chicago. Proponents of the flat tax argue it would make it easier for Americans to file their taxes. </p>
Credit Tim Boyle / Getty Images
As Rick Perry unveils his flat tax plan, we take a look back at the history of the flat tax, from Abraham Lincoln to Steve Forbes to 9-9-9.