I recently received an email message from a writer/teacher saying that she is seeing periods placed outside quotation marks “by all” of her (college-level) students, and has also received emails from colleagues that commit the same error. She questioned me because “so many people are using it this way.”
Periods and commas should be placed within quotation marks. Question marks and exclamation points are placed inside or outside quotation marks depending on whether they are part of the quotation. Colons and semicolons are placed outside quotation marks. The Chicago Manual of Style sees no reason to place a period or comma outside quotation marks except when it is “essential to make clear that the period is not included in the quoted material.” The example given is “Type in the code “W1.GH.748”.
These rules are used when writing for an American audience. British writers follow a rule of logic: periods and commas are placed either inside or outside quotation marks depending on whether they belong to the quoted material or to the sentence that includes the quoted material.
The Chicago Manual of Style calls the placement of periods and commas within quotations “a traditional style, in use well before the first edition of this manual (1906).” I found this comment in the Chicago Style Q&A: “Not that punctuation is necessarily logical, you know; sometimes it is simply based on convention.” The Associated Press Stylebook and the Publication Manual of the American Psychological Association agree with convention.
“The Yahoo! Style Guide” (writing and editing for the digital world) also places periods and commas within quotation marks, except for “a character or a string of characters that the user must type exactly,” but also suggests that the sentence be reworded so the placement of punctuation isn’t near the quotation marks or, alternatively, that boldface type be used for the string.
Beverly Ehrman edits fiction and nonfiction, magazine and newspaper articles, book proposals, educational and promotional materials, business documents, and website copy.