As an editor, it drives me crazy when I have to edit copy with double, or in some cases triple spaces between sentences. While I understand, many of us (me included!) learned to type on a type writer, the days of double spaces between sentences are gone.
On a typewriter, the characters are all monospaced. That means an “i” takes up the exact amount of space as an “m.” But on a computer the characters are proportional. Therefore, the double space is not needed.
An About.com article articulates this point. “The only time more than 1 space was used in a line of type was when we had to justify the line to the full margins.”— Madisonhank.
Seeing a full-page of text with double spaces looks strange. I can only imagine that a recruiter or Human Resources Manager would look at the cover letter written with double spaces and add it to the circular file (as in, throw it out). Using double spaces between sentences looks very out-of-place and immediately labels the author as unprofessional.
It took an effort to get my typing teacher’s voice out of my head (and the sight of those red marks indicating I didn’t double space), but it was worth it.