The question of whether to use one character space or two after a full stop is controversial for such a mild and unimportant topic.
It is also generational: kind of ‘baby boomers vs millennials’, as so many inter-generational disagreements are.
Don’t ask the Gen X-ers: they will probably just roll their eyes and shrug, as they do with all other disagreements between baby boomers and millennials.
In all seriousness it doesn’t matter too much. Although some advocates for each would argue that the other looks untidy, it is really a matter of fashion. Not grammar certainly, not usage, and barely style. Just fashion.
Which is not to say it is trivial. Content professionals can take it very seriously. Style guides carry opinions on it. It is worth knowing chapter and verse so you can make an informed decision.
But if someone who has decision-making authority over a piece of content – which could be a writer, a content director, an editor or even a publisher – really holds to the other one, let them. It really doesn’t matter. As long as they hold to it consistently and it is an unequivocal feature of their house or individual style guide.
Em-space and En-space
The origin of the double space is typographical. In the old days of manual typesetting it was possible to choose from a whole range of space sizes: from an Em-space (like the present day American Em-dash, the size of a capital letter M), to an En-space (letter N size, and a good approximation to our now-beloved single character space), and down to even smaller gaps.
Well into the twentieth century it was common practice for typesetters to use the Em-space after a full stop. Most typewriters, however, did not cater for such subtleties. The pragmatic choice for approximating an Em-space on a typewriter was to just type two single spaces.
This approach was actively taught on most typewriting courses right up to the advent of word processors. And, well beyond typing classes, it was assumed as a standard across the educational, publishing and commercial sectors, especially in more traditional organisations. Thus did whole generations grow up convinced that double spacing was the right and only way.
These days most style guides are clear that a single space is preferred. It is easier to monitor for consistency, and works better with modern fonts. Some people think it looks cleaner too, less messy, although if you are one of those brought up to think double spacing is the right and only way it is perilously hard to adapt.
The client is always right
I was one of those. But adapt I did, as a matter of pure pragmatism, when presented with a (millennial) client who was absolutely insistent on single spacing, and took me to task mercilessly when I strayed back into old habits. It did not last long. I am a dyed-in-the-wool ‘single’ man now.
Occasionally I yearn a bit for the old habit, and at times I almost persuade myself that double spacing after a full stop is actually more readable. But the Chicago Manual of Style demurs, stating that there is no evidence whatsoever for this. Most other style guides agree, though I did see that the American Psychology Association still waves the cutlass for the double space, slashing away drunkenly at the fresh air while its boat drifts out to sea, soon to be invisible on the horizon.
In the end that is an unfair analogy. The APA has every right to insist. It really doesn’t matter. It is a fashion thing, and old-fashioned fashions are not outlawed. The consistency principle is a hard one to counter, however. So probably, in the end, double character spacing will disappear from view, trounced by the pragmatic desire for uniformity which is at the heart of much publishing and editorial philosophy nowadays.
I won’t be too sad when that day comes.