Physical units are units used in measuring physical quantities. In specifying a unit, it is necessary to define an instance of that physical quantity and a way in which it can be compared in making a measurement. For example, the kilogram unit of mass is defined as the mass of a specified block of platinum-iridium. Other masses are measured by weighing them and comparing them, directly or indirectly, with this. Units are of two types: base units that, like the kilogram, have fundamental definitions, and derived units that are defined in terms of these base units. Various systems of units exist, founded on certain base units. They include the fps system (the imperial units, foot, pound, second), cgs system (centimeter, gram, second). For all scientific purposes, SI units have been adopted, which include seven base units: metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), candela (luminous intensity) and mole (amount of substance).