Double spaces was the norm on (almost always monospaced font) typewriters [EDIT: in English]. I assume it lingered on computers until sometime in the 80s but was shifting especially once proportional fonts for written material (other than things like code of course) became the norm.

I couldn't tell you when I switched.

Double spaces was the norm for English. I understand it is considered mostly obsolete nowadays.

It has not been the norm for German ever.

Edit: Related discussion: https://news.ycombinator.com/item?id=22975299

I was taught to use double spaces in highschool typing class (we used electric typewriters with the buffer off). In the ~35 years since I have only ever met a handful of people, all older than me, who did it in practice.

I continued through university while using a computer to type things up, I'm pretty sure my thesis had double spaces after periods, written in WP. Once I graduated I dropped it pretty quickly.

Was there an early version of Word's spell-check-on-the-fly that underlined double spaces, or am I miss-remembering?

>Was there an early version of Word's spell-check-on-the-fly that underlined double spaces, or am I miss-remembering?

I don't remember that and I was a fairly early user of Word going back to DOS days although I rarely use it any longer.

Apparently you could set up one or two spaces depending on your preference, however recent versions of Word now mark double spaces as an error by default.