\@ ¶Synopsis:
capital-letter\@.
Treat a period (or other punctuation) as sentence-ending, where LaTeX would otherwise think it is part of an abbreviation. LaTeX thinks that a period ends an abbreviation if the period comes after a capital letter, and otherwise thinks the period ends the sentence.
This example shows the two cases to remember.
The songs \textit{Red Guitar}, etc.\ are by Loudon Wainwright~III\@.
The first period ends the abbreviation ‘etc.’ but not the
sentence. The backslash-space, \ , produces a mid-sentence
space. The second period ends the sentence, despite it being preceded
by a capital letter. We tell LaTeX that it ends the sentence by
putting \@ before it.
So: if you have a capital letter followed by a period that ends the
sentence, then put \@ before the period. This holds even if
there is an intervening right parenthesis or bracket, or right single or
double quote, because the spacing effect of that period carries through
those characters. For example, this
Use the \textit{Instructional Practices Guide},
(a book by the MAA)\@.
will have correct inter-sentence spacing after the period.
The \@ command is only for text modes. If you use it outside
of a text mode then you get the error ‘You can't use
`\spacefactor' in vertical mode’ (see Modes).
All the above applies equally to question marks and exclamation points
as periods, since all are sentence-ending punctuation, and LaTeX
increases the space after each in the same way, when they end a
sentence. LaTeX also increases spacing after colon, semicolon, and
comma characters (see \spacefactor).
In addition: the converse case is a period (or other punctuation) that
does not end a sentence. For that case, follow the period with a
backslash-space, (\ ), or a tie, (~), or \@.
Examples are Nat.\ Acad.\ Science, and Mr.~Bean, and
(manure, etc.\@) for sale (note in the last one that the
\@ comes after the period but before the closing parenthesis).