Which unit is NOT commonly used to express length in the metric system?

In the metric system, length is primarily expressed using units such as millimeters, centimeters, and meters. These units are all part of the metric system, which is based on powers of ten, making it simple to convert between different units. Millimeters and centimeters are smaller units of measurement leading up to the meter, which is the base unit for length in the metric system.

On the other hand, feet are commonly used in the Imperial system, which is different from the metric system. As such, stating that feet is the answer highlights the distinction between the two measurement systems, emphasizing that feet do not belong to the metric system framework. Understanding these systems and their respective units is vital for proper measurement conversions and applications in fields that rely on precise measurements.