context: On August 15, 1971, the United States unilaterally pulled out of the Bretton Woods Accord. The US abandoned the Gold Exchange Standard whereby the value of the dollar had been pegged to the price of gold and all other currencies were pegged to the dollar, whose value was left to "float" (rise and fall according to market demand). Shortly thereafter, Britain followed, floating the pound sterling. The other industrialized nations followed suit with their respective currencies. Anticipating that currency values would fluctuate unpredictably for a time, the industrialized nations increased their reserves (by expanding their money supplies) in amounts far greater than before. The result was a depreciation of the dollar and other industrialized nations' currencies. Because oil was priced in dollars, oil producers' real income decreased. In September 1971, OPEC issued a joint communiqué stating that, from then on, they would price oil in terms of a fixed amount of gold.
question: What did the US withdraw from in 1971? 