In most systems of units, a single unit is defined as a base unit for the description of a specified quantity. Other units are then defined as fractions and multiples of this base unit. These are called **subsidiary units**.

For example, in the United States customary system of length units, the yard is the base unit. Other units, such as the inch or foot are defined as fractions of the yard, while yet others, such as the mile, are defined as multiples of the yard.