I think “right” or “wrong” are a tad strong for most of the cases
sited. But as a professional book designer and typographer, there’s
unquestionably “better” and “worse.”
For improved legibility, inter-sentence space should generally be a bit
greater than inter-word space.
Typewriters only had one distance they could travel. Either 1/10th of
an inch (“Pica”) or 1/12th (“Elite”). So the only way to add extra
space after a sentence was to double it. That’s way too much extra
space, but it was generally better than the alternative. The real
problem was that the words were too far apart, not that the sentences
were too close, but again, the fixed spacing was already an abominable
situation.
Proportional type, dating all the way back to Gutenberg, would
generally use 1/3rd or 1/4th of the height of type type as the
inter-word spacing. This would usually work out to about the width of a
lower case “t” or “l”.
When setting modern (by which you may also read “all type before
typewriters” as well) proportional type in fully justified form (left
and right margins both even), the spaces must be stretched out on a
line-by-line basis to fit. Really good typesetting programs (and really
good typesetters sticking little bits of lead between their words (and
I’ve done that, too)) will add more of the space between sentences than
between words, so as the line stretches, the inter-word space to
inter-sentence space ratio actually changes. (Take a look at a narrow
newspaper column sometime.)
More sophisticated approaches to space will ignore a user’s attempt to
sprinkle extraneous space in. Less sophisticated ones might allow it,
and even treat them as individual spaces, stretching both of them
during expansion. {shudder}
The fact that both the MLA Guidelines and the Bedford Handbook
encourage poor typography is regrettable. (“If you cannot type
appropriate punctuation, e.g. an em-dash or en-dash, please use
appropriate substitutions. For both dashes, substitute a pair of
hyphens, which, like true dashes, are typed without adjacent spaces.”
There’s still software out there that will happily wrap a line between
the two hyphens. Ick!) Nevertheless, if you’re submitting a paper to an
institution that expects or requires that, then to not follow them is
wrong, even if the legibility of the submission is better.
What it all boils down to is “Putting two spaces after a period at the
end of a sentence is an artifact left over from the days when the
typewriter was the prevalent text-making tool. Unless you have a
specific reason or requirement to do otherwise, it’s preferable to put
only one space between sentences.”
For breaking text into sentences, sometimes I find it easier to work
backwards. Also, only very colloquial writing will have a one-word
sentence, so you can solve all “Mr./Dr./Ph.D.” cases by the fact that
if a word starts with a cap and ends with a period, it’s not a
sentence. For a more sophisticated approach that’s still not too
complex to program, check the final word of a sentence against a
dictionary. If it’s found there without a final dot, then you’re almost
certainly looking at the end of a sentence. If it isn’t, then is it
found anywhere else in the document without a dot? If not, then you’re
probably looking at an abbreviation. (My mail program uses a monospaced
font. If I thought most readers would read it with a proportional font,
I’d have typed “Ph. D.” above, since it should have a thin space before
the D.)