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System of units; S.I A complete set of units, both base and derived, for all kinds of physical quantities is called a system of units. In S.I units, seven quantities are chosen as base; their units are defined and accepted as standard units   SUPPLEMENTARY UNITS This class contains two units: the SI unit of plane angle, the radian, and the SI unit of solid angle, the steradian Considering that plane angle is generally expressed as the ratio between two lengths and solid angle as the ratio between an area and the square of a length. The supplementary units radian and steradian are dimensionless derived units These supplementary units may be used in expressions for derived units to facilitate distinguishing between quantities of different nature but the same dimension Prior 1960, three measuring systems were in practice