Emacs uses the presence of double spaces after periods as a marker that that particular sequence of text is the end of a sentence. It doesn't insert them by def

Emacs uses the presence of double spaces after periods as a marker that that particular sequence of text is the end of a sentence. It doesn't insert them by default, but rather interprets their insertion in a particular way. See: sentence-end-double-space, a variable, and sentence-end, a function, both defined in ‘paragraphs.el’. Other editors made this same interpretation of existing, common practice.