This is something I was unsure about for years (often seeing the divergent usage) and now there is an exquisite explanation:
- “The period next to a quotation.” – inside of the mark in the American grammar.
“The period next to a quotation”. – outside of the mark in British grammar and now more common on the Internet.
Without knowing the rule, I used mostly the American style of quotation, it seems more logical and graphically clean in “containing” the quote, a period belongs to a sentence not to the entire passage. Ben Yagoda in Slate – A punctuation paradigm is shifting.
Further reading: