The monetary system of the Western World is based on the agreement on the establishment of the International monetary Fund （IMF）, which was made at Bretton Woods, New Hampshire （USA）, in 1944. The two principal purposes of the IMF are to promote exchange stability and to help member countries overcome temporary difficulties in their balance of payments. Under the IMF system, the US dollar is the key or leading currency. Each member country of the IMF undertakes to establish a parity of its currency in terms of gold and the US dollar and to keep fluctuations of the dollar exchange rate -- or, more exactly, the spot rate within the limits of the band or fluctuation margin. Originally, the band was 2 per cent （1 per cent on either side of the parity）; in December 1971 it was widened to 4.5 per cent （2.25 per cent on either side）, the upper and the lower intervention point. Each member country's central bank must, if necessary, buy or sell dollars to keep the dollar rate from falling below the lower or rising above the upper intervention point. In this manner, the other currencies, among themselves, cannot deviate from one another by more than double the fluctuation margin , or 8 per cent.西方世界的货币体系是根据1944年在美国新罕布什尔州布雷顿森林制定的关于建立国际货币基金组织协议而形成的。国际货币基金组织的两个主要宗旨是促进汇率稳定和帮助会员国克服国际收支方面的暂时困难。在国际货币基金组织体系下，美元是关键货币。国际货币基金组织的每一个成员国承担着稳定本国货币与黄金和美元平价的责任，并使美元汇率——或者，更准确地说——美元现汇汇率的波动限制在规定的幅度内。最初，波动幅度为2%（在平价上下各1%）；1971年12月，这一波幅扩大为2.25%，即上下干预点。每个成员国的中央银行在必要时应买进或抛售美元，使美元汇率下降的幅度不低于最低点，或者上升的幅度不超过最高点。以这样的方式，其他货币之间相互偏离的幅度不能超过波动幅度的两倍，或者说，不能超过8%。The parities can be changed if need be, but changes in excess of 10 per cent require the approval of the Fund. Parity changes have to be made to correct an imbalance resulting from a permanent improvement or deterioration of a country's competitive position. If, for example, a country has a lower rate of inflation than the other countries, its exports will increase and its imports decline. The growing demand for the currency of this country will cause its external value to appreciate, but since under a system of fixed or pegged parities，exchange rate fluctuation are limited----a point will be reached from which the currency will become increasingly undervalued. If, on the other hand, a country has a higher inflation rate than the other countries, the opposite will be true and the currency of this country will become increasingly overvalued.Such an imbalance is further aggravated by a speculative run out of the overvalued currency into the undervalued one. Therefore, it will eventually be necessary to upvalue the undervalued currency and/or to devalue the overvalued one.如果需要，平价可以调整，但调整幅度超过10%，须经基金组织批准。进行平价调整的目的是为了纠正由于永久性地改善一国的竞争状况，或一国竞争地位恶化而造成的国际收支失衡。比如，如果一国的通货膨胀率低于其他国家，该国的出口将会增加，进口将会下降。对该国货币需要的增加将使其货币的对外价值上涨，但是，因为在固定汇率或钉住关键货币的制度下，外汇汇率的波动是受到限制的 —— 上涨到某一点后，该货币就会不断贬值。另一方面，如果一个国家的通货膨胀率高于其他国家，就会出现相反的情况，该国的货币就会不断的升值。这种国际收支不平衡的状况会因为投机活动而进一步加深，即由于价值高估的货币大量流入货币价值低估的国家所造成。因此，提高低估货币的价值和/或降低高估货币的价值，最终成为必然