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The internal currency system is BTC (XBT), which is a virtual currency whose rate is determined by the results of trades between the users. No traditional economic crisis can have a significant impact on the exchange rate, and in the long term it has been constantly growing. The total issuance is limited - no more than 21,000,000 BTC can circulate (it is estimated that this figure will be reached in 2033). Thus, the currency's value increases over time like precious metals do. For instance, one of bitcoin's forum users bought a pizza in 2010 for 10,000 coins, and by the end of 2013 this amount cost over $10,000,000!