- Modern Language Association (MLA) Style
- "Leave one space after a period or other concluding punctuation mark, unless your instructor prefers two spaces. Whichever spacing you choose, be sure to use it consistently throughout your paper. See the MLA’s formatting guidelines for more information."
- https://style.mla.org/number-of-spaces-after-period/
- Publications Manual of the American Psychological Association (also known as the "APA style")
- "allows for two spaces in draft manuscripts but recommends one space in published work"
- "Although the usual convention for published works remains one space after each period, and indeed the decision regarding whether to include one space or two rests, in the end, with the publication designer, APA thinks the added space makes sense for draft manuscripts in light of those manuscript readers who might benefit from a brief but refreshing pause."
- http://blog.apastyle.org/apastyle/2009/07/on-two-spaces-following-a-period.html
- AP Style Guide Cheat Sheet
- "Use only one space after the end of a sentence. Period."
- https://www.codot.gov/business/grants/safetygrants/documents/APStyleGuideCheatSheet.pdf
- Chicago Manual of Style
- US Government Printing Office Style Manual
- Section 2.49, "A single justified word space will be used between sentences (key one space when typing). This applies to all types of composition."
- https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2016/pdf/GPO-STYLEMANUAL-2016.pdf
- Typography for Lawyers
- Grammar Girl
- "If you learned to type on a typewriter, you’re going to hate what I say next: Do not put two spaces after a period. Don’t do it. Just use one."
- https://www.quickanddirtytips.com/education/grammar/two-spaces-after-a-period
- https://www.quickanddirtytips.com/education/grammar/how-many-spaces-after-a-period
And a general article about using One Period after sentences, https://slate.com/technology/2011/01/two-spaces-after-a-period-why-you-should-never-ever-do-it.html
Additional sources, https://en.wikipedia.org/wiki/Sentence_spacing_in_language_and_style_guides