Customary units do not have primary definitions. They are legally defined as fractions of metric standards. When one researches the science of measurement, you discover this is very significant and a very clear indication that no organization, including the U.S. government, is supporting customary units.

What does this mean?

Example: What is a a statute mile? (note nautical mile has a different definition and multiple definitions for the same base unit-mile-is another issue with customary units) A mile is 5280 feet.  What is a foot?  A foot is 12 inches.  What is an inch?  An inch is 1/36 of a yard.  What is a yard?  A yard is 3 feet or 36 inches. What is foot?  What is an inch?….. It is this circular logic that leads to nothing- no primary definitions within customary units.

We consider this circular logic more supporting evidence that customary units are NOT a system of measurement.  A system, after all, is a set of interconnecting parts forming a complex whole. Customary units are an assortment of disparate, unconnected units based on historical artifacts and colonial legacies chiefly supported through our continued teaching of them and reinforced by our consumption habits.

How did the US government resolve our measurement problem?  It legally defined customary units as fractions of metric standards!!

Example: Legally and scientifically, what is a mile? A mile is 1.609344 kilometers.  What is a kilometer?  A kilometer is 1000 meters.  What is a meter? A meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

Bam, there is a clearly definition end point and it is based on a scientific constant!  The metric system is indeed a true system, designed to facilitate easy mathematical and scientific calculations.

Our question:  

If customary units are legally defined as fractions of metric standards, then how is it more advantageous for our students to intuitively think in customary units instead of directly thinking in metric units?