Jupiter Images/liquidlibrary/ ThinkstockMany myths swirl around the metric system and U.S. involvement with it. First, the relationship between the metric system and the United States dates back to the 18th century, not the 1970s. Second, all countries have either fully adopted or legally sanctioned the International System of Units, or SI, the modern form of the metric system. Metric Association Web site, contributor Dennis Brownridge identifies at least nine different meanings for the unit we know as a \"ton\": short ton, displacement ton, refrigeration ton, nuclear ton, freight ton, register ton, metric ton, assay ton and ton of coal equivalent. To understand why the U.S. doesn't use the metric system in its commercial activities and everyday life, it helps to look at a brief history of how the European system of measurement came to U.S. soil.