Last updated on Fri Jul 9 00:09:14 CEST 2021.
The practice of double-spacing is to add two spaces instead of one after a period and the next sentence. It was widespread in the past and particularly associated with typewriters and fixed-width fonts. From there, it spread to computer programmers, and I would guess that it is primarily programmers who keep the practice alive today.
This is an example of double-spacing. Note the extra space between sentences. A bit jarring to the unfamiliar eye.
The arguable benefit of double-spacing is that it is clearer to visually see where one sentence starts and the other begins. This is in my opinion undoubtable. (The question is whether the sentence breaks stick out in the good kind of way or in the sore thumb kind of way.)
Another big benefit is technological. Double-spacing makes it easy not only for humans to recognize sentence breaks, but also for computers. It allows ambiguous cases -- like "Mr." -- to be resolved unambiguously.
However, the practice varies between languages. It seems that double-spacing is particularly popular in English, while not at all as widespread elsewhere. Because of this, none of the technological benefits of double-spacing can be applied by software developers, if they have any regard for non-English languages.
So, at the end of the day, it comes down to the following questions:
The second question is a matter of taste. I somewhat lean towards considering it ugly.
The first question, however, is hypothetically a perfectly empirical one. Does double-spacing really help all that much in recognizing sentence breaks? It would be interesting to see some studies on this topic.
In the absence of such data, I personally suspect the answer to be: a little, but not enough to be significantly useful. Sentences are already visually set apart, both by the presence of the period and the capitalization of the next sentence.
Here is the same sentence again, but single-spaced this time:
This is an example of double-spacing. Note the extra space between sentences. A bit jarring to the unfamiliar eye.
Sure, the sentence breaks are not as jarring. But they're still pretty obvious, are they not?
In the last couple of days, I've stopped double-spacing my English prose -- to see how it feels, it nothing else. One thing I've noticed is that many common UNIX programs rely on and/or enforce double-spacing. Here are the programs I've had to patch so far: