Physics is a quantitative science, so every statement about nature ultimately rests on measurement. Measuring a physical quantity means comparing it with a basic, arbitrarily chosen, internationally accepted reference standard called a unit. The result of any measurement is therefore a number, called the numerical measure, accompanied by a unit; a number without a unit conveys nothing. Although the number of physical quantities in nature appears very large, they are interrelated through laws and definitions, so we need only a small set of independent quantities, called fundamental or base quantities. These are length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. The units chosen for them are the fundamental or base units. All other quantities, such as velocity, force or energy, are derived quantities, and their units, obtained as combinations of base units, are called derived units. Together the base units and derived units form a complete set known as a system of units. This economy, with few base units generating all the others, keeps measurement consistent worldwide, simplifies communication among scientists, and allows equations to be checked through their units. Understanding the distinction between base and derived units is the foundation for dimensional analysis, which appears later in this chapter.