The United States uses the United States customary units, which is a system of measurement units commonly used in the United States and most U.S. territories. The United States customary system developed from English units that were in use in the British Empire before the U.S. became an independent country. The system uses units such as feet, inches, pounds, ounces, etc. . In commercial activities, as well as for personal and social use, customary units are used, while in science, medicine, many sectors of industry, and some government and military areas, metric units are used. The International System of Units (SI), the modern form of the metric system, is preferred for many uses by the U.S. National Institute of Standards and Technology (NIST). However, the United States is one of three nations (along with Liberia and Myanmar) that have not adopted the metric system as their official system of weights and measures. The reasons for not adopting the metric system are mainly due to the cost and time required to change the entire infrastructure of the country, as well as the belief that the United States should keep its particular system, setting it apart from other countries and symbolizing its status as a leader rather than a follower.