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    • Spell out numbers up to and including nine; use numerals for 10 and above, including ordinals (Eighth Conference on Severe Local Storms, 12th Symposium). Numerals are always used with abbreviated units of measure: 25 km, 1 m, etc. If the unit of measure is not abbreviated, rule 1 above applies: 12 months, six kilometers, etc.

    • Use numerals with very large dollar amounts and large fractions: $3 million, 6.28 million dung beetles. Spell out whole numbers from one to nine, except when you wish to convey a higher level of precision (editor's choice): about two million refugees, but they counted 8.5 million species.

    • Use an en dash within a number range: 6–12 months, but when preceded by "from," insert "to":  from 6 to 12 months. (Also see: en dash)

    • Hyphenate numbers and units in adjectival phrases: six-month term, 20-year appointment, 9-km length, etc.

    • Insert a zero before decimal points (in numbers less than 1) except in statistical correlations, and even there if it would look peculiar not to.

    • To make numbers plural, add an s (with no apostrophe): 1940s, 16s, ones, fives, 10s, etc.

    • Use commas in four-digit and larger numbers except those expressing wavelengths: 3,124 years but 4120 A. Four-digit page numbers have no comma: pg. 5769; five digits or more take a comma for legibility: pg. 11,275.

    • Use numerals for numbers used as nouns: a factor of 2. Zero is an exception: value of zero.

    • Avoid negative exponents whenever possible: 1.7 X l0-5 photons cm-2 s-1 keV-1 becomes 1.7 X l0-5 photons/(cm2s keV). Note, however, that wave numbers (the reciprocal of wavelength) are expressed with negative exponents.

    • Money—See money

    • Units of measure: Style depends on the audience:

      • Media and public outreach: use U.S. customary first (then put metric equivalent in parens).

      • Research community, scientific agencies, etc: fuse metric units only


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