Sat on my floor tonight reading old school typography books. they were a bit dusty, but i was looking for something specific. i got frustrated with not finding the sentence i remembered reading sophomore year of college so i opted for the wikipedia route. i’ll find it someday, but this will have to do for now.

Sentence spacing with single, but enlarged, spaces was widely employed before the 20th century. Double spacing[3] (sometimes referred to as English spacing) came into widespread use with the introduction of the typewriter in the late 19th century.[4]It was felt that with the monospaced font used by a typewriter, “a single word space … was not wide enough to create a sufficient space between sentences”[5] and that extra space might help signal the end of a sentence.[6] This caused a widespread change in practice. From the late 19th century, printers were told to ignore their typesetting manuals in favor of typewriter spacing; in the 1890s, Monotype and Linotype operators used double sentence-spacing[7] and this was widely taught in typing classes.[8]

With the advent of the computer and the widespread use of proportional fonts, double sentence spacing became obsolete.[9] These proportional fonts now assign appropriate horizontal space to each character (including punctuation marks), and can modify kerning values to adjust spaces following terminal punctuation, so there is less need to manually increase spacing between sentences.[5] From around 1950, single sentence spacing became standard in books, magazines, newspapers, and webpages.[10] This was French spacing—a term synonymous with single sentence spacing until the late twentieth century. However, many still believe that double spaces are correct. The debate continues..

told you so