Formatting text and using the convention of putting 2 spaces after a period

I've noticed that when I select a block of text (written English
prose, not code) and press gq to reformat it, vim will change some of
the spaces after a period into two spaces, as is the convention among
some typists.

Is this intended, and can it be changed?

Personally, I prefer to keep just one space after a period, but I
notice that it does happen consistently - sometimes it will change
the spaces and sometimes not.

--

quit When the quit statement is read, the bc processor
is terminated, regardless of where the quit state-
ment is found. For example, "if (0 == 1) quit"
will cause bc to terminate.
-- seen in the manpage for "bc". Note the "if" statement's logic

.--. oo
(____)//
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~'

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