The customary system was the first system to evolve. It was based on the human body, starting with ancient Egyptian's use of the cubit, the measurement of the arm from the elbow to the end of the fingertips. This was followed by the foot, based on the length of a foot. Standard weights were used in Ancient Rome and in England, based on the weight of a single grain. Capacity, or volume, evolved from containers that were used to hold grains.
Metric measurements were introduced many years later, starting in the late 1500s and 1600s in France. Measurements were based on the Earth, starting with the meter, which is one ten-millionth of the distance from the equator to the North Pole. Weight and capacity were derived from the centimeter, which is one hundredth of a meter. All of the types of measurements are from the base ten number system and use the same prefixes such as kilo, deci, milli, and centi. The metric system was first used in the science and medical fields. It was designed to be a more uniform system.
The metric system has the definite advantage when using conversions within the system. A cubic centimeter holds one milliliter of water and weighs one gram. This links distance, volume, liquid capacity and weight. All conversions are based on multiples of ten as there are 1000 centimeters in one meter, 1000 grams in a kilogram and 1000 milliliters in one liter. Even temperature is based on 100 degrees between freezing and boiling in a Celsius thermometer.
It is more difficult to remember the conversions within the customary system. For distance, there are 12 inches in one foot, three feet in one yard, and 5,280 feet in a mile. For weight there are 16 ounces in a pound and 2,000 pounds in a ton. For liquid capacity there are two cups in a pint, two pints in a quart and four quarts in a gallon. There is also no relationship between measuring distance, weight and capacity. For temperature, there are 212 degrees between the melting point and boiling point of water.