It is generally accepted that the practice of putting two spaces at the end of a sentence is a carryover from the days of typewriters with monospaced typefaces. Two spaces, it was believed, made it easier to see where one sentence ended and the next began. Most typeset text, both before and after the typewriter, used a single space.
“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, describing typesetting before digital type
Today, with the prevalence of proportionally spaced fonts, some believe that the practice is no longer necessary and even detrimental to the appearance of text.
With monospaced typefaces every character takes up the same amount of space on the page. M uses the same amount of space as i. With proportionally spaced fonts, the characters take up an amount of space relative to their actual width - the i needs less space than the M.
The use of proportionally spaced type makes two spaces at the end of a sentence unnecessary (if they ever were). The extra spacing is often distracting and unattractive. It creates holes in the middle of a block of text — trapped white space on a smaller scale.
