16th amendment
i'm not really a history buff. or a law buff. but i recently looked this up. the 16th amendment of the constitution is the one about income taxes. i never really understood this one long ago in civics class. the constitution specifies certain types of taxes must be spent on each state according to its fraction of the total population. prior to the 16th the government had the right to tax income. but the supreme court ruled that this would be one of those taxes that had to be spent proportionally. which apparently would have been an accounting nightmare. it was easier to change the constitution. which is what happened. the 16th amendment gives congress the right to tax income without regard to the aforementioned restriction on how the money is to be spent.