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.