THE YIELD OF THE FEDERAL INDIVIDUAL income tax increased from $55.4 billion in fiscal year 1966 to an estimated $103.3 billion in fiscal year 1973. This is an increase of $47.9 billion over a period during which personal income from production—henceforth to be identified as adjusted personal income—rose $357.2 billion, suggesting a marginal rate on the increased income—or built-in flexibility—of about 13.5 percent. The built-in flexibility so computed is only the roughest of approximations, since it does not allow for the changes in the tax structure that took place during the period. Although tax rates were the same at the beginning and the end of the period, a number of major structural changes that affect revenues in both directions were enacted in 1969 and 1971. The purpose of this paper is to measure the built-in flexibility of the federal individual income tax as it applies to incomes received in 1973 and later years.