The traditional convention in American English
is for commas and periods to be included inside the quotation marks, regardless of whether they are part of the quoted sentence, while the British style places them in or outside of the quotation marks according to whether or not the punctuation is part of the quoted phrase. The American rule is derived from typesetting while the British rule is grammatical (see below for more explanation). Although the terms American style and British style are used, it is not as clear cut as that because at least one major British newspaper prefers typesetters' quotation
(punctuation inside) and BBC News
uses both styles, while scientific and technical publications, even in the U.S., almost universally use logical quotation
(punctuation outside unless part of the source material), due to its precision.
As with many such differences, the American rule follows an older British standard. Before the advent of mechanical type, the order of quotation marks with periods and commas was not given much consideration. The printing press required that the easily damaged smallest pieces of type for the comma and period be protected behind the more robust quotation marks.
The typesetter’s rule
was standard in early 19th century Britain, and the U.S. style still adheres to this older tradition both in everyday use and in non-technical formal writing. The grammatical rule
was advocated by the extremely influential book The King’s English
, by Fowler and Fowler.