Where did the common act of double-spacing after a period come from?

The practice of double-spacing following a period began with typing which used no proportional fonts. That is, every letter and punctuation mark floats in the middle of the same space. "W" and "I" are extreme examples of letters that are vastly different in width but you can still recognize words because you don't read letter by letter. You read words by their shapes. The period is not a word. It is just a tiny speck easily lost on the page. The double-space helped readers distinguish the end of sentence. Proportional spacing does not have this problem. Every letter fits in an appropriate sized space. Indeed, inserting a double space at the end of a sentence while typing in a word processor using a proportional font will screw up the algorithm that controls spacing

I was taught to double space after a period in school. During my civilian career, I mentored third and fourth graders in subjects they were weak in. One of my third graders taught me that the new English rules called for one space after a period.

SGT (Join to see), when I took typing in high school, double spacing was required after a period and a colon. Single spacing was required after a comma and a semicolon. Those were the typing rules from way back when.

I was taught to double-space after periods and colons. I like CPT Jack Durish explanation. I would add the use of double-space after the period may be related to setting "hot type" or "cold type" for printing documents before computerized printing came onto the scene. In the case of either hot or cold type setting, the period is on a small, narrow slug of metal placed after the last letter of the last word of the sentence. In the printing process, the raised type letters and punctuation are inked with a roller and the paper is pressed against the inked characters, hence the name "printing press," run by people called "pressmen." Placing a slug for two blank spaces (no ink) after the period helped keep it clear in the printing process. (Hot type machines (linotype machines) used hot metal (lead or aluminum) to make rows of reversed face letters that were set into blocks of type for printing. Cold type is usually set by hand from wood boxes with different size letters and punctuation on little metal pieces. Watching an experienced type setter do his or her job is seeing an artist at work as their hands fly over the boxes, selecting the needed type pieces, and placing them to create words, sentences, and documents.)

When people used typewriters it prevented overlap of characters though it's said typist were mimicking earlier print settings.

I don't know that it is strictly a Gen X vs. millennial situation. APA formatting suggests using two spaces after periods. I got used to it while writing papers for my schoolwork.

Type writers. But however today's modern view is not to double space because computers can auto adjust spacing.

Not an easy question to answer. It is one with a storied history. http://www.businessinsider.com/never-put-two-spaces-after-a-period-2014-1

Depends on the writing discipline you use. Most modern academic use APA formatting designations... Which still call for two spaces after the period... Same with Chicago 6 and MLA. Most systems that use single space are rogue disciplines that are unrelated to established. However, when you write a dissertation, the board will instruct you in what format you must use.

I was taught NOT to double space after a period in school. It wasn't until I started writing reports during a TRADOC assignment that I was told to double space... It just looks weird to me...