**The International System of Units (SI)**
For many years, the units used in science were, in general, metric units, which were developed in France in the eighteenth century. Metric units are related decimally, that is, by powers of ten. This relationship is usually indicated by a prefix attached to the unit.
In 1960 the General Conference of Weights and Measures proposed a revised and modern metric system called the International System of Units (SI from the French Le **S**ystem **I**nternational d´Unites). The system has seven **base units** and two **dimensionless **units (formerly called **supplementary** units), all other units being derived from these nine units.There are 18 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters. Decimal multiples of the units are indicated by a set of prefixes; whenever possible a prefix representing 10 raised to a power that is a multiple of three should be used.
**Base and *dimensionless SI units **
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