Here’s a topic I hear a lot: one space or two after a period? (Or, if you’re English, after a full stop.) And everyone quotes typewriters as the source of the two-space style but rarely know why.
When printing using lead blocks for each character, there is more than one type of space. In the early 1900’s, the prevailing style was to use an m-space after a period. (This is a space that was is as wide as the letter ‘m’ in the same font.) By the 1990’s this had changed to either an n-space (as wide as the letter ‘n’) or a quad-space (as wide as it is tall).
Typewriters can’t do variable width letters. And just as the m-dash was simulated by two dashes, the m-space was simulated with two spaces.
Now since the m-space, n-space, and quad-space are all wider than the interword space, there should still be a wider gap between sentences than between words. So, yes, there should only be one space after a period … but only if it’s the right space.
A few years ago, I asked in the office I work at if it should be one space or two in our documents. Everyone agreed it should be one space … but then would look around furtively and whisper, “But it looks better with two.” This is borne out by a recent study at Skidmore College (by Johnson, Bui, and Schmitt) using eye tracking, that showed there was an increase in readability with two spaces.
With a word processor, it’s easy to convert double spaces to single using the global replace function. It’s harder to convert the other way if you have abbreviations. So, personally, I draft with two spaces and then, if necessary, remove them before submitting.