[Tex/LaTex] the proper use of \@ (i.e., backslash-at)

best practicescapitalizationspacing

In this answer, \@ is used after a period and before an \xspace, presumably to indicate that the preceding period (in "etc.") was not sentence-final.

In these tips, \@ is used before a period and after capital letters, to indicate that the preceding capital letters are sentence final.

Since \@ is rather difficult to search for online, I thought I'd try to ask here: what exactly does \@ do, and how do you use it properly/consistently? For instance, how should you input the following farcical sentence?

Languages like JS, HTML, etc. were not used by King Henry III.

EDIT: I'm accepting Barbara's answer below as it (and the comments on it) indicate the initial intent of \@: "the period which follows is sentence-final". Its counterpart is \  (slash-space), which says "the period which precedes is sentence-medial". Egreg's answer explains in far more detail exactly how \@ and \spacefactor work to cause the effects seen by \@ and \  after lower- and upper-case letters and punctuation. Both are very helpful answers.

Two correct answers to my made-up sentence above is

Languages like JS, HTML, etc.\ were not used by King Henry III\@.
Languages like JS, HTML, etc.\@ were not used by King Henry III\@.

The former is arguably "better style", while the latter demonstrably produces the correct result even if it seems less consistent about placement of \@ versus periods. And a safe but slightly redundant answer blindly adds \@ before every comma: 

Languages like JS\@, HTML\@, etc.\ were not used by King Henry III\@.

This last one is amenable to macro-izing: \def\acronym#1{\textbf{#1}\@\xspace} (or whatever style you'd prefer), where the \xspace disappears if the next character is punctuation, and the \@ ensures that subsequent punctuation thinks it follows a lowercase word and not an uppercase initial.

Best Answer

i find the cited answer rather confusing, if not out-and-out backwards. \@ before punctuation says that the period does fall at the end of a sentence. to quote from the latex manual (p.170):

\@ Causes an "end-of-sentence" space after punctuation when typed before the punctuation character. Needed only if the character preceding the punctuation character is not a lowercase letter or a number.

so your farcical sentence is input as

Languages like JS, HTML, etc.\ were not used by King Henry III\@.

only one \@, and that after a capital I.

Update:
I have been reminded, rather ignominiously, that \@ is not defined in classic TeX. If used, the result is "! Undefined control sequence." (The corresponding command for TeX to render an end-of-sentence space following a period following an uppercase letter is \null.) \@ is LaTeX only.

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