Why Stock Splits Might Occur Why the Swiss Franc is a Frequently Traded Currency

Why the Currency Market is Really a Market of Dollars and Anti-Dollars

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While the stock market is truly a market of stocks, the currency market is really a market of dollars and anti-dollars. The central reason why this is so is that the dollar serves as the reserve currency for the world's central banks. Therefore, when speculators are bullish on the dollar, capital will flow from all the major currencies into the greenback and vice versa when the sentiment reverses.
Source: http://www.investopedia.com/articles/forex/05/CurrencyCross.asp