Like the national or fiat currencies traded in the forex market, cryptocurrencies trade in pairs relative to other cryptocurrencies or to fiat currencies like the U.S. dollar or the European Union's euro. When cryptocurrencies trade relative to other cryptocurrencies, the resulting pair is called a crypto cross. The number of available cryptocurrencies, and hence the number of cryptocurrency pairs, changes frequently as new cryptocurrencies are issued through their initial coin offerings or ICOs. The current number of existing cryptocurrencies is estimated at over 2,500 and rising. When newly issued cryptocurrencies are modelled on Bitcoin, perhaps with some changes or improvements, they are called altcoins. A popular example of an altcoin is Litecoin, which was one of the first cryptocurrencies issued after Bitcoin.