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English-language numerals

There are two main English-language systems of number names: the Americann system (also used in Canada) and the European system (used in most Germanic and Romance languages). The European system is the older one and is also called the British system, because it was used in Great Britain at one point. Britain and Australia have recently largely switched to the American system. The American system was originally invented by the French, adopted by the United States, and then abandoned by France.

The principal difference between the two systems is the meaning of the word "billion" and its multiples: in the European system a billion is a million million (1012), a trillion is a million billion (1018), and so forth; a thousand million (109) is called a milliard, a thousand billion (1015) a billiard etc. In the American system a billion is a thousand million (109), a trillion is a thousand billion (1012), and so forth. See more below.

After the million mark, the European number system is based on millions, the American number system is based on thousands. The prefix of the number name corresponds to the power of the base number. e.g. bi means 2, tri means 3, quadri means 4, etc.

In the European system (see Chuquet!), each number is a power of a million, i.e. (n)illion = (one million)n = 10n×6.
one billion = (one million)2 or 1012;
one trillion = (one million)3 or 1018;
one quadrillion = (one million)4 or 1024;
one quintillion = (one million)5 or 1030;
one sextillion = (one million)6 or 1036;
one septillion = (one million)7 or 1042;
one octillion = (one million)8 or 1048;
one nonillion = (one million)9 or 1054;
one decillion = (one million)10 or 1060;
one undecillion = (one million)11 or 1066;
one duodecillion = (one million)12 or 1072;
one tredecillion = (one million)13 or 1078;
one quattuordecillion = (one million)14 or 1084;
one quindecillion = (one million)15 or 1090;
one sexdecillion = (one million)16 or 1096;
one septendecillion = (one million)17 or 10102;
one octodecillion = (one million)18 or 10108;
one novemdecillion = (one million)19 or 10114;
one vigintillion = (one million)20 or 10120;
...
one centillion = (one million)100 or 10 600.

In the American system, each number is a power of a thousand (the prefix and the number of power will match only when the first 1000 is factored out), i.e. (n)illion = 1000 × 1000n or 10(n+1)×3.

one billion = 1000 * 10002 or 109;
one trillion = 1000 * 10003 or 1012;
one quadrillion = 1000 * 10004 or 1015;
one quintillion = 1000 * 10005 or 1018;
one sextillion = 1000 * 10006 or 1021;
one septillion = 1000 * 10007 or 1024;
one octillion = 1000 * 10008 or 1027;
one nonillion = 1000 * 10009 or 1030;
one decillion = 1000 * 100010 or 1033;
one undecillion = 1000 * 100011 or 1036;
one duodecillion = 1000 * 100012 or 1039;
one tredecillion = 1000 * 100013 or 1042;
one quattuordecillion = 1000 * 100014 or 1045;
one quindecillion = 1000 * 100015 or 1048;
one sexdecillion = 1000 * 100016 or 1051;
one septendecillion = 1000 * 100017 or 1054;
one octodecillion = 1000 * 100018 or 1057;
one novemdecillion = 1000 * 100019 or 1060;
one vigintillion = 1000 * 100020 or 1063;
...
one centillion = 1000 * 1000100 or 10 303.

NOTE: Most of these number names are not used in ordinary writing. The "-illion"s above "quintillion" are almost nowhere to be seen, except perhaps in lists of number names such as this one. Larger numbers are referred to as powers of ten using scientific notation, for instance, as "1025" (read "ten to the twenty-fifth"), etc. "Googol", however, is not uncommonly used for rough approximations.

Indian English additionally uses the words lakh and crore to denote 100 thousand (lakh) or ten million (crore, i.e. 100 lakhs). e.g.,
• Kaun Banega Crorepati, the Indian version of Who Wants to Be a Millionaire
• "Faced with the worst drought conditions in three decades, the Andhra Pradesh government has asked for an assistance of Rs 2160 crore from the central government..."

Umpteen is used to mean a large number. Usually it refers to the number of times something has happened, rather than a number of objects. Usually however the number referred to is less than a hundred.

Zillion and gazillion are fictitious very large numbers. The terms 'a (ga)zillion' and '(ga)zillions' are used informally, meaning very many, as in the Zillions of Games software product.

See How to name numbers in English