The monetary system implemented at Bretton Woods in 1944 made the US dollar the centre of the world economic system, with 43 other countries' currencies linked to it via fixed exchange rates. However, once the US government broke its promise to redeem dollars in gold at $35 per ounce on August 15, 1971, expansion of the supply of dollars was no longer constrained, and like many currencies before it, the lack of monetary discipline led to inflation through which the value of the dollar has fallen by about 98%. The “oil shock” of the 1970s led to the introduction of the “petro-dollar” system whereby Saudi Arabia, then the largest oil producer, agreed to accept only US dollars in payment for its oil in exchange for the US government's pledge to defend it. This shored up demand for the fiat US dollar, enabling it to survive until its now approaching endgame.