As I got into writing and began learning about standard format and various practices, I got the impression that one should double space after a period. This goes with the idea I gathered, whether right or wrong, that a manuscript composed on a computer or other word processing device should as closely as possible, resemble one constructed on a typewriter. And since it is the practice to double space after a period when using a typewriter, it should also be done when using a computer. Thus, I have always and continue to include two spaces after a period or other sentence ending punctuation in my manuscripts and indeed in most things I write. I also understand that that little bit of extra room afford by the second space can come in handy for one editing by hand on paper. Only once have I ever changed to a single space in those locations and that was to meet specific requirements of an agency or publisher I was submitting my work to. (And if I'm on Twitter, I'll use only one space because of the limited number of characters allowed.)
These days there seems to be a push to convince us old fashioned folks that two spaces after a period is wrong and that we shouldn't do it. They say computers and modern fonts elimate the need, but even with a modern proportional font, I like that little bit of extra space between sentences. And as nearly as I can figure out, that was why the practice originally started. Typewriters are non-proportional, alloting the same amount of line space to an "I", an "l" or a "j" as to a "W" an "M", or an "X." Overall that made words appear oddly proportioned and even with a period, the gap between words at the end of a sentence and the beginnings of the next may have been difficult to distinguish. Hence the use of two spaces to better emphasize the full stop and the beginning to the next sentence.
In published works it's common to only use one space, but honestly, there are times where to me the two sentences look a bit crowded. I've also heard that publishers/printers use a single space in order to save paper. Of course cost is a big concern and if they can print the book using 350 pages instead of 351, they are going to do so. And if they can save that page by including a single space after every period, they will do so.
Last night I got to wondering how much is saved in the typical book by using a single space after a period or other sentence ending punctuation. I went through a book by a well known contemporary author and on a random selection of pages, counted the number of places where there could have been two spaces after a period. I came up with an average of sixteen places per page. I went through one of my books a while ago and looked at 6 random pages. each fully covered with text. Again these averaged out to sixteen places where the printer could have used two spaces per page. That would add something like, "sixteen places," about what is quoted, to each page.
Manuscript for the third Stone Island Sea Story open to a random page. You probably can't see it, but there are two spaces after each period or other sentence ending punctuation.
What I realized and found interesting, however, is that much of the time, these extra spaces could be asborbed at the tail end of each paragraph, where the line is incomplete and there is room for more text before going to the next line, next paragraph. And we should remember that in most published formats, chapters often end part way down the page, and that page is left blank, with the next chapter starting on the next page. That new chapter often starts well down the page, with a lot of blank page existing between the end of one chapter and the next.
So I'm now wondering if that push to use only a single space between sentences really save any pages at all. The book I happened to look at first is written in many, many, very short chapters. I believe more pages are used here because of a chapter ending halfway down the last page and the next starting a third of the way down the next, rather than the use of two spaces after a period. Its quite possible that a particular page might bleed over to the next when using two spaces, but that will be negated at the end of the chapter, the end of the next paragraph, or the end of the next page.
I will continue to add two spaces after a period or other sentence ending punctuation, and wonder if printers and publishers shouldn't consider it in order to make their products a bit easier to read.