\frenchspacing & \nonfrenchspacing ¶Synopsis, either:
\frenchspacing \nonfrenchspacing
\frenchspacing causes LaTeX to make spacing after all
punctuation, including periods, be the same as the space between words
in the middle of a sentence. \nonfrenchspacing switches back
to the default handling in which spacing after most punctuation stretches
or shrinks differently than a word space (see \spacefactor: Extra space after punctuation).
In American English, the typesetting tradition is to increase the
space after punctuation more than the space between words that are in
the middle of a sentence. Invoking \frenchspacing (the command
is inherited from plain TeX) switches all spaces to be treated the
same, which is the convention in essentially all other languages (to
the best of our knowledge), not just French. The command name is
another inherited convention.
If your LaTeX document specifies the language being used, for
example using the babel package, the necessary settings
should be taken care of for you.