A study showed that it's hard to measure any difference in reading speed and there is no difference in comprehension.

The difference in reading speed only applies to people who are used to double-spaces, there is no difference in reading speed single-spaced vs. double-spaced for people who are conditioned to read single-spaced.

> Although the type of spacing following punctuation marks did not seem to have an effect on those individuals who type with one space after a period, those who type with two spaces after a period had greater reading speed when paragraphs were presented in the same way in which they type: with two spaces following periods and one space following commas.

I'd argue this data suggests the double-spaces are a waste of space that accomplishes nothing except slightly handicapping people who become accustomed to it and then read text without it.

https://link.springer.com/article/10.3758/s13414-018-1527-6

>slightly handicapping people who become accustomed to it and then read text without it.

Or maybe slightly boosting when reading double spaced? (I didn't read the article yet, my plane is just starting).

Books are typically not monospace, which is where the two-space rule really applies.

Yep, I use vimwiki where it's all monospace, and two spaces between sentences is a massive readability benefit there. I also type it when making comments here because these input boxes are also monospace, even though the extra space isn't displayed after posting.

No, the two-space rule was used in books a ~century ago. That's what I'm comparing with.

The habit was adopted for monospace typewriters precisely to try to match book typesetting.

Then the books dropped it, but it persisted in typing habits.

Then once Mac/Windows allowed anyone to use proportional fonts, we were taught to use a single space again.

They do. The end of a sentence is not the same as the space after an abbreviation. There is a semantic difference, and it's useful.