\@ (LaTeX2e unofficial reference manual (November 2018))

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#### 19.5.1 `\@`

Synopsis:

    capital-letter\@.

Treat a period 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. By default, in justifying a line LaTeX adjusts the space after a sentence-ending period (or a question mark, exclamation point, comma, or colon) more than it adjusts the space between words (see \spacefactor).

This example shows the two cases to remember.

    The songs \textit{Red Guitar}, etc.\ are by Loudon Wainwright~III\@.

The second period ends the sentence, despite that it is preceded by a capital. We tell LaTeX that it ends the sentence by putting `\@` before it. The first period ends the abbreviation ‘`etc.`’ but not the sentence. The backslash-space, `\` , produces a mid-sentence space.

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 a text mode. If you use it outside of a text mode then you get ‘`You can't use `\spacefactor' in vertical mode`’ (see Modes).

Comment: the converse case is a period ending an abbreviation whose last letter is not a capital letter, and that abbreviation is not the last word in the 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 before the closing parenthesis).

Unofficial LaTeX2e reference manual