I learned to do it because old-school unix vi would identify your sentences better for sentence-based moves if you had 2 spaces after the period. I'm actually trying to unlearn it now because you don't need it for any kind of vim/neovim and I read on some ultrapedantic typesetting website that it's wrong for some reason I don't quite remember now.
In any case it's definitely not specifically American (neither am I) and all the other "forensics" that people are trying based on vocab etc are somewhat of a stretch also. eg non-Americans use "gotten" non British people use "British" English (eg people from Ireland or from former British colonies for the most part) non-Americans use "ize" sometimes for spellings (I could never be bothered to learn the few exceptions needed to spell "ize/ise" words correctly in the British style and worked enough for American companies who wanted US spelling as a house style for my personal spelling to be even remotely consistent and certainly not indicative of where I am from.
As a British person, despite having been taught that the '-ise' suffix is 'proper' English, I have made some effort to unlearn this habit, as it was never really based on any etymological roots anyway. Here's what Wiktionary has to say about it[1]:
Many English verbs end in the suffix /aɪz/. Historically, this has been spelled -ize on words originating from Greek (for example baptize, Hellenize), while -ise has been used, especially in -vise, -tise, -cise and -prise, on words that came from French or Latin roots (for example surprise, supervise). In the 19th century, it became common in the United Kingdom (due to French influence)... to use -ise also on words that had historically been spelled -ize (hence baptise, Hellenise). However, the... Oxford English Dictionary continue to use the spelling -ize on Greek words, and -ize has always been the spelling used in the United States and Canada on such words.
The whole debate becomes rather moot when it is considered that Ancient Greek didn't use the Latin script, so both '-ize' or '-ise' would have looked distinctly foreign to a Greek author two thousand years ago. Horace so succinctly noted that "Captive Greece took captive her savage conqueror and brought civilisation to barbarous Latium", but he might have been a little less glowing if the orthography of Greek loanwords was as heated a debate in Rome as it is in contemporary Britain!
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.
I didn’t think double spaces after a full stop is an American thing. We were taught that in the 70s back when typewriters were still a thing. And I can’t break the habit today.
I was taught typing on electric typewriters in junior high, and yes 2 spaces after a period.
Which is strange, because my first typing experience was on Apple IIe's in grade school. I don't recall any typing instruction back then, so probably my double space habit comes from the electric typewriter instruction.
Dr.[space]Pepper is a soft drink.[space][space]This is a new sentence. Actually, "Dr Pepper" doesn't use a period so the point is void, but there's definitely some potential semantic (and display) differences between the periods in (eg) N.A.S.A. and the periods at end of a sentence. Not quite as straightforward as you might think.
The books go into it, and LaTeX has \. for periods that are not sentence-stops (there's even more, as word-breaking and line-breaking come into play, as you don't want to end a line with Dr. when it's part of a name, etc.
