The market in which participants are able to buy, sell, exchange and speculate on currencies.  Foreign exchange markets are made up of banks, commercial companies, central banks, investment management firms, hedge funds, and retail forex brokers and investors. The forex market is considered to be the largest financial market in the world.

Investopedia Says:
Because the currency markets are large and liquid, they are believed to be the most efficient financial markets. It is important to realize that the foreign exchange market is not a single exchange, but is constructed of a global network of computers that connects participants from all parts of the world.

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