USD Dollar (US Dollar)
After the World War II US dollar became the leading currency of the world. Today it occupies about 50-61% of central banks reserves that makes 1 trillion US dollars in total. US dollar is a universal payment means, an object of investments and an asylum-currency for other countries in case of financial and political crises.
All profit for national debt securities in United States is paid off in time and is tax-free that creates financial and economic stability of USA, attracts foreign governments and private investors.
While trading with other currencies US Dollar in most cases is considered to be generally accepted base currency.