subclass_of si derived unit
The ___MASK30___ ___MASK52___ is an internationally agreed decimal ___MASK52___ of ___MASK29___ . It was originally based on the and the introduced by the French First Republic in 1799 , but over the years the definitions of the metre and the kilogram have been refined , and the ___MASK30___ ___MASK52___ has been extended to incorporate many more units . Although a number of variants of the ___MASK30___ ___MASK52___ emerged in the late nineteenth and early twentieth centuries , the term is now often used as a synonym for " SI " or the " International System of Units " the official ___MASK52___ of ___MASK29___ in almost every country in the world . A ___MASK89___ is a mark , ___MASK72___ , or ___MASK9___ that indicates , signifies , or is understood as representing an ___MASK84___ , object , or ___MASK19___ . Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different concepts and experiences . All communication ( and ___MASK7___ processing ) is achieved through the use of symbols . Symbols take the form of words , sounds , gestures , ideas or visual images and are used to convey other ideas and beliefs . For example , a red octagon may be a ___MASK89___ for " STOP ". On a map , a blue line might represent a river . Numerals are symbols for numbers . Alphabetic letters may be symbols for sounds . Personal names are symbols representing individuals . A red rose may symbolize love and compassion . The variable ' x ', in a mathematical equation , may symbolize the position of a particle in space . A ___MASK52___ of ___MASK29___ is a collection of units of ___MASK29___ and rules relating them to each other . Systems of ___MASK29___ have historically been important , regulated and defined for the purposes of science and ___MASK50___ . Systems of ___MASK29___ in modern use include the metric ___MASK52___ , the imperial ___MASK52___ , and United States customary units . ___MASK68___ is a ___MASK88___ that can exist as a magnitude or multitude . Quantities can be compared in terms of " more ", " less ", or " equal ", or by assigning a numerical value in terms of a ___MASK40___ . ___MASK68___ is among the basic classes of things along with quality , substance , change , and relation . Some quantities are such by their inner nature ( as number ), while others are functioning as states ( properties , dimensions , attributes ) of things such as heavy and light , long and short , broad and narrow , small and great , or much and little . A small quantity is sometimes referred to as a quantulum . ___MASK29___ is the assignment of a number to a characteristic of an object or event , which can be compared with other objects or events . The scope and application of a ___MASK29___ is dependent on the context and discipline . In the natural sciences and engineering , measurements do not apply to nominal properties of objects or events , which is consistent with the guidelines of the " International vocabulary of metrology " published by the International Bureau of Weights and Measures . However , in other fields such as statistics as well as the social and behavioral sciences , measurements can have multiple levels , which would include nominal , ordinal , interval , and ratio scales . The International ___MASK52___ of Units ( SI ) specifies a set of seven ___MASK26___ units from which all other SI units of measurement are derived . Each of these other units ( SI derived units ) is either dimensionless or can be expressed as a product of ( positive or negative , but usually integral ) powers of one or more of the ___MASK26___ units . For example , the SI derived unit of area is the square metre ( m2 ) , and the SI derived unit of density is the kilogram per cubic metre ( kg / m3 or kg m  3 ) . The degree Celsius ( see the table below ) has a somewhat unclear status , and is arguably an exception to this rule . The names of SI units are written in lowercase . The symbols for units named after persons , however , are always written with an uppercase initial letter ( e . g . the symbol for the hertz is `` Hz '' ; but the symbol for the metre is `` m '' ) . The International ___MASK52___ of Units (; abbreviated as SI ) is the modern form of the metric ___MASK52___ , and is the most widely used ___MASK52___ of ___MASK29___ . It comprises a coherent ___MASK52___ of units of ___MASK29___ built on seven ___MASK26___ units . The ___MASK52___ also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units . A ___MASK40___ is a definite magnitude of a quantity , defined and adopted by convention or by law , that is used as a standard for measurement of the same quantity . Any other value of that quantity can be expressed as a simple multiple of the ___MASK40___ .