## How Many? A Dictionary of Units of Measurement © Russ Rowlett and the University of North Carolina at Chapel Hill Table of Contents About the Dictionary Using the Dictionary Using Numbers and Units

In science, there are definite rules of syntax for using numbers and units. This dictionary tries to follow those rules.

Large numbers: How many is a billion? In America, the word "billion" means the number 1 000 000 000, or 109. In Britain, this number is (or was) traditionally called "one milliard" or "one thousand million," and "billion" means the number 1 000 000 000 000 or 1012, which is what Americans call a "trillion." Most other European languages have similar expressions, such as mil millones for 109 and billón for 1012 in Spanish.

The Dictionary uses the American definitions for large number names. Although the General Conference on Weights and Measures endorsed the European names in 1948, the American definitions seem to be gaining ground in Britain and perhaps elsewhere. See Names for Large Numbers for more on this question.

There's always interest, especially among school children, in knowing the names of still larger numbers: quadrillions, quintillions, etc. These names are listed in on the Names for Large Numbers page.

Decimal Points and Numbers: In English-speaking countries, the decimal marker (decimal point) is the period. In continental Europe and most other places, the decimal marker is the comma. The International System (SI) allows either one to be used; the Dictionary uses the period. Since the comma is often a decimal marker, the SI recommends that large numbers, like the billions above, be represented as groups of three digits separated by narrow spaces. The Dictionary observes this convention. The SI also allows large numbers to be written without any grouping, as in 158346, but it does not allow the use of commas or any other grouping marks other than narrow spaces.

Scientific Notation: Many authorities, including the U.S. NIST (Guide for the Use of the International System of Units, sec. 7.9), also recommend that in scientific notation (when numbers are represented using powers of ten), the exponent of the 10 should be a multiple of 3. For example, the distance from the Earth to the Sun should be stated as approximately 149.6 x 106 km, not 1.496 x 108 km. This rule fits the structure of the prefixes used for metric units. The Dictionary follows this rule for most numbers, but not for stating the ratios between CGS and MKS units.

Symbols (Abbreviations): Following the rules of the International System, the Dictionary uses no periods after abbreviations or symbols for units, and never uses an "s" on an abbreviation or symbol to indicate a plural.