What is a yard?
A yard is 3 feet or 36 inches. In the US you mostly see it on football fields, which run 100 yards between the goal lines, and in the UK short road distances are still signed in yards.
The yard is traditionally the distance from the nose of King Henry I of England to the thumb of his outstretched arm, though its length varied from place to place for centuries. It was only fixed internationally in 1959, at exactly 0.9144 meters.
Nearly every other country uses the meter instead. In the US, bulk soil and mulch are still sold by the cubic yard, which is 27 cubic feet.
What is a millimeter?
A millimeter is a thousandth of a meter (1/1000) which is the SI (International System of Units) unit of length. It is normally used to measure small lengths like the thickness of a sheet of paper or the dimensions of a small object.
One millimeter is approximately equal to 0.03937 inches (about 1/25th). Presicely there are 25.4 millimeters in an inch and it is often used in science and engineering. It is used in countries that have adopted the metric system.
You may come across millimeters when measuring the size of electronic components, jewelry or even the thickness of a fingernail.