A yard is currently defined by the metric system. Metrology is the discipline of measuring. It concerns itself with ensuring that everyone has the same defini

A yard is currently defined by the metric system. Metrology is the discipline of measuring. It concerns itself with ensuring that everyone has the same definition of measuring units. For a long time, the yard was based on physical standards. i.e. Somewhere there sat a physical rod 1 yard long that all other rods were measured against to define the yard. The meter was defined this way until 1960 too. The unfortunate problem with physical standards is that they change. A physical rod is different lengths at different temperatures and will, of course, gradually get shorter as it's handled and wear occurs on its ends. The meter is now defined in terms of how fast light can travel in a given period of time. That period of time is defined in terms of hyperfine ground state level transitions of a Cesium atom. Yes, it's complex. However, while a physical standard changes, these standards are built into the laws of the universe and are thus reproducible and fixed for all time. For this reason, the yard is currently defined as 0.9144 meters in the U.S.. No matter how the metric system defines the meter, a yard is 0.9144 meters. So, the U.S. really has gone metric, at least where it counts. The average American is just ignorant of the fact. P.S. Even the metric system is not completely free of physical definitions... yet. The kilogram is still based on a physical prototype. However, there are proposals to change that to something based on physical constants. Once this is done, the metric system will be defined entirely in terms of physics.