http://www.dailywealth.com/136/the-only-sure-thing-we-know-about-gold-and-the-u-s-dollar

"With the U.S. unable to eliminate its trade deficit, Nixon was faced with the stark reality of another dollar devaluation. He opted instead to close the gold window on August 15, 1971, ending dollar-for-gold convertibility. The dollar was suddenly off the gold standard, and half of U.S. gold holdings had disappeared. The greenback began to "float," meaning it wasn't tied to any standard and could be printed at will.  So how's it done since then?"