Metric prefix  A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit. While all metric prefixes in common use today are decadic, historically there have been a number of binary metric prefixes as well. Each prefix has a unique symbol that is prepended to the unit symbol. The prefix "kilo-", for example, may be added to "gram" to indicate "multiplication" by one thousand: one kilogram is equal to one thousand grams. The prefix "milli-", likewise, may be added to "metre" to indicate "division" by one thousand; one millimetre is equal to one thousandth of a metre. Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six of these dating back to the system's introduction in the 1790s. Metric prefixes have also been used with some non-metric units. The SI prefixes are metric prefixes that were standardized for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 1991. Since 2009, they have formed part of the International System of Quantities. They are also used in the Unified Code for Units of Measure (UCUM)  The BIPM specifies twenty prefixes for the International System of Units (SI). Each prefix name has a symbol that is used in combination with the symbols for units of measure. For example, the symbol for "kilo-" is k, and is used to produce km, kg, and kW, which are the SI symbols for kilometre, kilogram, and kilowatt, respectively. Except for the early prefixes of "kilo-", "hecto-", and "deca-", the symbols for the multiplicative prefixes are uppercase letters, and those for the fractional prefixes are lowercase letters. There is a Unicode symbol for micro "µ" for use if the Greek letter "μ" is unavailable. When both are unavailable, the visually similar lowercase Latin letter "u" is commonly used instead. SI unit symbols are never in italics. Prefixes corresponding to an integer power of one thousand are generally preferred. Hence 100 m is preferred over 1 hm (hectometre) or 10 dam (decametres). The prefixes "hecto-", "deca-", "deci-", and "centi-" are commonly used for everyday purposes, and the centimetre (cm) is especially common.