Why is the number 1 trillion have 18 zeros in Great Britain and only 12 in the us?

Answer 1

See the related link for a complete explanation of long and short scale countries and the history of the naming convention for large numbers.

The long and short scales are two of several different large number naming systems used throughout the world: Short scale is the English translation of the French term échelle courte.[1] It refers to a system of large number names in which every new term greater than million is 1,000 times the previous term: billion means a thousand millions (109), trillion means a thousand billions (1012), and so on. Long scale is the English translation of the French term échelle longue. It refers to a system of names in which every new term greater than million is 1,000,000 times the previous term: billion (from bi and million) means a million to the power of two or a million millions(1012), trillion (from tri and million) means a million to the power of three or a million billions(1018), and so on. Your question is actually factually incorrect as the UK (since 1974) now uses the short scale numbering system as does the US. However, there might be some legacy use of the long scale usage.