This article is from the alt.usage.english FAQ, by Mark Israel firstname.lastname@example.org with numerous contributions by others.
According to William F. Phillips (email@example.com), in the days
when printing used raised bits of metal, "." and "," were the most
delicate, and were in danger of damage (the face of the piece of
type might break off from the body, or be bent or dented from above)
if they had a '"' on one side and a blank space on the other. Hence
the convention arose of always using '."' and ',"' rather than '".'
and '",', regardless of logic.
Fowler was a strong advocate of logical placement of punctuation
marks, i.e. only placing them inside the quotation marks if they
were part of the quoted matter. This scheme has gained ground,
and is especially popular among computer users, and others who
wish to make clear exactly what is and what is not being quoted.
Logical placement is accepted by many more publishers outside than
inside the U.S.
Some people insist that '."' and ',"' LOOK better, but Fowler
calls them "really mere conservatives, masquerading only as