Once upon a time, students who learned how to type on a typewriter — you’ve heard of those, right? — were trained to add two spaces after a period under penalty of death. Not getting a gold star was tragic.
- Look! This laptop won’t even let me type two spaces without marking the error.
- Using two spaces looks odd. Wide open. Bizarre.
- Using a single space looks clean. It’s tidy. Oops, I did it again!
The good people who created modern word-processing software decided that computers would be able to slightly expand the space after a period, to save its users from that extra thumb action. This has been true for decades, now that we have moved from monospace (one character per space) to proportional space (lovely kerning that makes the words fit together nicely). Save yourself unnecessary work and join the twenty-first century: one space after all sentence-ending punctuation!
Irish and Anglo-Saxon monks developed spacing between words at about the same time that they were singing plain-chant in monasteries: 600 CE and beyond. It made reading much easier, and became the norm by the Renaissance. There was usually an extra space after the period to separate sentences; the use of two spaces wasn’t standard until the 1970s.