British versus American style
There are two major styles of English punctuation: American (commonly followed also in Canada) and British (commonly followed also in Australia and New Zealand). Over the years, these two styles have converged. The few major differences that remain are described below.
American style uses double quotes (“) for initial quotations, then single quotes (‘) for quotations within the initial quotation.
“Economic systems,” according to Professor White, “are an inevitable byproduct of civilization, and are, as John Doe said, ‘with us whether we want them or not.’”
British style uses single quotes (‘) for initial quotations, then double quotes (“) for quotations within the initial quotation.
‘Economic systems’, according to Professor White, ‘are an inevitable byproduct of civilization, and are, as John Doe said, “with us whether we want them or not”’.
The above examples also show that the American style places commas and periods inside the quotation marks, even if they are not in the original material. British style (more sensibly) places unquoted periods and commas outside the quotation marks. For all other punctuation, the British and American styles are in agreement: unless the punctuation is part of the quoted material, it goes outside the quotation marks.
Mr., Mrs., and Ms. all take periods in American English. In British English, the periods are omitted.
British usage dictates a period between the hours and minutes when writing the time (e.g., 10.30). American usage dictates a colon (e.g., 10:30).
Though not necessarily a matter of punctuation, there is one important distinction between American and British usage when it comes to dates. American usage puts the month first, followed by the day, and then the year. Hence, 12/5/2010 means December 5, 2010, in American usage. The British practice (followed in most of the world) is to put the day first, followed by the month. Hence, 12/5/2010 means May 12, 2010, in British usage. The International Organization for Standardization (ISO) has established the YYYY-MM-DD format, in which December 5, 2010, would be written 2010-12-05. Whether this will catch on with American writers remains to be seen. In the meantime, writing out the month will avoid confusion.