Put two spaces after a period

What actually happened

The reason was mechanical. A typewriter gives every character the same width, so an 'i' occupies as much space as an 'm', and sentence breaks get visually lost in the even texture. Two spaces restored the pause. Professional typesetting, which had proportional fonts, never adopted the habit, and books printed throughout the twentieth century use single spacing.

Word processors made the workaround obsolete around 1985 by bringing proportional fonts to everyone, but typing curricula kept teaching the rule for another fifteen years, so an entire generation learned muscle memory for a machine they would never use. Word's April 2020 update to the editor, flagging the double space as a suggested correction, is the tidy end date, and it caused a genuinely disproportionate amount of internet rage.

There is one real study worth mentioning honestly. A 2018 paper in Attention, Perception, & Psychophysics found a small reading-speed benefit from two spaces, but only in a monospaced font, and only for people who already typed two spaces, which is roughly the conditions under which the rule made sense in the first place. In the proportional fonts everyone actually reads, there is no case for it.