The meaning of billion and other large numbers may depend on the country in which they are used or, more probably these days, the age of the writing in which they appear. The original meanings were easy to decipher, following the bi, tri, quad sequence (2, 3, 4, etc) that is found in other words (bilingual, tricycle, quadruped, etc). Thus billion was 2 millions multiplied or 1 000 000² (10¹²), trillion meant 1 000 000³ (10¹⁸), quadrillion meant 1 000 000⁴ (10²⁴), and so on. European scientists began to change this by a factor of 10, and Americans followed, so that 1 billion became 1 000 000 000 1000 million, or 10⁹), and the rest of the sequence was similarly changed to 10¹² and 10¹⁸. Today, 1 billion is taken to mean 1000 million (10⁹), but its meaning may be ambiguous where it is encountered in writing from the past. Some scientists and communicators, especially when their audience is likely to be international, prefer to use the explicit 1 thousand million or 1 million million to make their meaning unambiguous. See also Numbers and units