If you grew up in the United States, you probably learned the United States Customary System (USCS) for weights and measures. USCS terms like inches, feet, pounds, and miles are derived from the British Imperial System, steeped in a long history of application and use. Any introduction to the metric system may have muddied the measurement waters, adding unfamiliar words and awkward conversions to your school day.
While Americans continue to use the USCS, most of the world has shifted to the metric system. The metric system, or International System of Units (SI), was introduced in France during the late 18th century and includes seven foundational units of measurement, the most common of which include meters and kilograms. SI has, according to its proponents, more utility and universality than other systems, making it the preferred system for weights and measures across the globe.
If this is true, why does the United States remain one of the few countries not to use the metric system as its primary system of measurement? The reasons Americans don't use the metric system are complicated, combining political and economic considerations with nationalist pride, but if you look closely, you can see the influence of SI in the United States. Whether or not this will eventually come full-scale remains to be seen, but here's what has shaped the use of the metric system in the United States so far.