A unit prefix is a specifier or mnemonic that is prepended to units of measurement. Units of various sizes are commonly formed by the use of such prefixes. The prefixes of the metric system, such as kilo- and milli-, are well known examples. These represent multiplication by powers of ten. In the information technology field it is common to use binary prefixes, which are based on powers of two. A number of prefixes, have been used or proposed by various sources, but not adopted by any standards organisation. There are also prefixes which official support have been dropped.