Historically, many different systems of units have been used, where a system of units is defined as a collection of units of measurement with rules that relate them to each other. A unit of measurement is a defined magnitude of a quantity that it used as a standard for measurement for the same kind of quantity, such as measurements of length, weight, and volume.
In the past, many systems of measurement were defined on a local level, and could be based on factors as arbitrary as the length of a king's thumb. While this may work on a local level, when considering trade, as well as science, having systems of units based on units that others may not be able to relate to or understand makes interaction difficult. As such, the development of more universal and consistent systems developed over time. Today, some of the systems of units in use include the metric system, the imperial system, and the United States customary units.
The International System of Units (SI) is the standard metric system that is currently used, and consists of seven SI base units of length, mass, time, temperature, electric current, luminous intensity, and amount of substance. Although SI is used almost universally in science (including in the US), some countries such as the United States still use their own system of units. This is partly due to the substantial financial and cultural costs involved in changing a measurement system compared to the potential benefit of using a standardized system.