The issue of one space vs. two spaces at the end of a sentence has long been a hot topic for discussion on this site. For typeset material, one space is the rule. A recent Slate article dove into the one-two space controversy boldly asserting "If you type two spaces after a period, you're doing it wrong." No exceptions. Interesting read although it may make your blood boil if you're a hardcore two-spacer (which I'm not).
There are all kinds of reasons people give for using two spaces instead of one. Here's a tiny, tiny sampling from my readers:
Habit:
i couldn't break the habit. i need 2! thank you :) - dd
Taught by the nuns, two spaces after a period. - detailerjohn
I've used two spaces for 49 years, and will continue to do so. I get annoyed when web forms strip out that extra space - like I expect this one will. - Roger K.
I have always typed with two spaces after sentence ending punctuation and will continue to do so regardless of what anyone else says. - Don K
Over all these years I have trained my brain to look for that extra space that demarks a writer's thought, and my thumb to create it. I suppose I could re-train it, but I feel no urgency to do so. - mg
Readability:
Two spaces are easier to read. Especially if the reader has any kind of eyesight challenges. - gingerer
Sometimes single spaced sentences tend to look like run-ons. A comma gets one space and a period deserves two. - twentysevenfour
Two spaces helps the eye to know when a sentence stops and another begins. - James
Age:
Whether to use one or two spaces depends on the age of your target audience: if they grew up with computers (born around 1970 or later) one space is good; if they are older, two spaces works better. - Leo
(Hmm... so those of us born before 1970 who know that ONE space works better are what, young at heart?)
Appearance:
It just looks so much better - therefore, effective! - Kathleen C
Two spaces is much more pleasing. One space creates to [sic] much gray space on the page while two spaces is generally more appealing to the reader. - colonel
Two spaces of course. Visually looks better... - JMaryAnn
I actually prefer the patterns in negative space that are created by the double space. It gives each paragraph a unique shape, rather than looking like daunting, monotonous blocks. - AJJR
Abbreviations: (distinguishing the end of a sentence period from a middle of the sentence period on an abbreviation such as Mr. or S.W.A.T., etc.)
Personally, I prefer two spaces after a sentence for the simple reason that it makes it easier to differentiate between abbrevation periods and sentence ending periods. - kittykat3376
When should punctuation be followed by two spaces? When it ends a sentence. Especially with periods, it's not just about font or whatever else you can dream up. Imagine a sentence where you have a period that is NOT ending the sentence. Such as, "... and directed the matter of A.P.A. Where generally..." Now did I mean A.P.A. end of the sentence? or did I mean there is such a thing as "A.P.A. Where"? Do you want people to guess your meaning? Use two spaces to be clear, one space if you just don't really care. - COS
(Well, if I read "...and directed the matter of A.P.A. Where generally..." I would assume two sentences unless you've thrown out the generally accepted rules of capitalization and threw in that Where (instead of where) for kicks and grins and confusion.)
Old vs. New (from both sides of the issue):
I see no advantage whatsoever to single spaces between sentences and I don't think that I should be asked to just adjust to the new pseudo-standard since there is such a simple solution that produces much more readable text. Return to two spaces!!! - Norman D
If you are so set on using two spaces perhaps you need to seek therapy for post-traumatic stress disorder so you can catch up to the digital age. Don't hold yourself back embrace change! - yuppett
Two spaces is SO mid-20th Century. Give your thumb a break and go with one. - Phineas P
It's time to change to single space - don't let the old habit of double space make a dinosaur out of your presentations. - Richard G.
We should all adapt to changes that occur around us. The time of typewriters is slowly coming to its end and the next generation of publishing has arrived. Everytime we type two spaces after a sentence, some program will strip off these two spaces and we would have wasted most of our precious time encoding two spaces that would in the end display as single space. - Journeyman of Publishing
Do you have a different reason for preferring two spaces over one -- or one instead of two? What's your preference?
Another pressing punctuation issue you might want to delve into: Prime Time to Ax the Apostrophe


Although I have adapted to one space, I think two spaces are more readable and make a nicer looking document. Less crowded.
Hey, let’s all switch over to to Twitter-Speak, since we’re obviously intent on accommodating the least educated among us.
At first, I had a hard time breaking the habit, but now I deffinately feel that one space is better, especially with on-screen communication. I don’t think “two-spacers” are wrong, but it is becoming “old school”
As a parent of a Kindergartener and 2nd Grader, I can’t help being frustrated when reading assignments are sent home, with the purpose of testing and timing fluency, yet there is only one space (a narrow one at best) between sentences. I cannot fault my child for running sentences together when the brain obviously needs this space to visually anticipate the break. I understand the special needs of graphic designers and professional typesetters, but the Elementary school student is not concerned with these. There must be basic rules of writing or children cannot be taught. Is there no longer an authority on this subject?
Before you comment, I only used one space in my above comment to make a point. Doesn’t it look better with two spaces? You be the judge…
As a parent of a Kindergartener and 2nd Grader, I can’t help being frustrated when reading assignments are sent home, with the purpose of testing and timing fluency, yet there is only one space (a narrow one at best) between sentences. I cannot fault my child for running sentences together when the brain obviously needs this space to visually anticipate the break. I understand the special needs of graphic designers and professional typesetters, but the Elementary school student is not concerned with these. There must be basic rules of writing or children cannot be taught. Is there no longer an authority on this subject?
Well, isn’t that interesting! I used two spaces between sentences on that last version, and one space was cut out by About.com in translation! How am I supposed to teach my kids? They won’t even let them “carry” the numbers in addition anymore. Guess it’s time for ME to go back to school.
I just asked my 2nd grader, and it turns out we’re ALL wrong. It’s not ONE space, not TWO spaces, but a “FINGER SPACE”. How does that translate into HTML? I rest my case. Last comment on this subject!
No. One space was cut out by your browser. It’s HTML. That’s how it works. That’s how it’s always worked. Unless you put in the coding for a non-breaking space, the browser will render only one space no matter if you use 2, 3, 10, or 20. No conspiracy by About.com.
See Why don’t space characters show on Web pages: http://webdesign.about.com/od/beginningtutorials/f/blfaqwhitespace.htm