The United States has a highly developed mixed economy. It is the worlds largest economy by nominal GDP and net wealth and the second-largest by purchasing power parity (PPP). It also has the worlds eighth-highest per capita GDP (nominal) and the tenth-highest per capita GDP (PPP) in 2018. The U.S. has the most technologically powerful economy in the world and its firms are at or near the forefront in technological advances. The U.S. dollar is the currency most used in international transactions and is the worlds foremost reserve currency, backed by its economy, its military, debt reimbursement, and the petrodollar system.Several countries use it as their official currency, and in many others, it is the de facto currency. The largest U.S. trading partners are China, Canada, Mexico, Japan, Germany, South Korea, United Kingdom, France, India, and Taiwan. The U.S. is the worlds largest importer and the second-largest exporter. It has free trade agreements with several nations, including NAFTA, Australia, South Korea, Israel, and few others which are in effect or under negotiating stage.The nations economy is fueled by abundant natural resources, a well-developed infrastructure, and high productivity. It has the seventh-highest total-estimated value of natural resources, valued at $45 trillion in 2016. Americans have the highest average household and employee income among OECD member states,and in 2010, they had the fourth-highest median household income, down from second-highest in 2007. By 1890, the United States had by far the worlds most productive economy. It is the worlds largest producer of petroleum and natural gas. In 2016, it was the worlds largest trading nation as well as its second-largest manufacturer, representing a fifth of the global manufacturing output. The U.S. not only has the largest internal market for goods, but also dominates the trade in services. U.S. total trade amounted to $4.2 trillion in 2018. Of the worlds 500 largest companies, 121 are headquartered in the U.S. The U.S. has the worlds highest number of billionaires with total wealth of $3.013 trillion.