The international system of units (SI = Système International d’Unités) was formally agreed by the 11th General Conference on Weights and Measures in 1960. The SI System is made up of 7 base quantities with their corresponding base units and symbols: length (metre m), mass (kilogramme kg), time (second s), electric current (ampere A), thermodynamic temperature (kelvin K), amount of substance (mole mol), luminous intensity (candela cd). The numerous additionally existing quantities are classified differently, as e.g. quantities with own names and symbols that are defined on the basis of the SI base units and called “coherently (logically) derived SI units” (e.g. the quantity Force F with the unit Newton N, derived from the ratio of the SI base units kilogramme, metre, second: 1 N = 1 kg·m/s²; and the quantity pressure p with the unit Pascal Pa, generated by the ratio of N/m²: 1 Pa = 1 N/m²) as well as units outside the SI System but allowed as they fulfill certain criteria (these include e.g. minute min, hour h, day d for the quantity time; bar bar, millimetres of Mercury mmHg, etc. for the quantity pressure).
