One or two spaces after a sentence?

Two spaces after a sentence is a holdover from the days before computers and desktop publishing. Typewriters used mono-spaced fonts where every letter had an equal amount of horizontal space. Adding two spaces after a sentence made mono-spaced lines easier to read. However most every font on a PC or Mac uses proportional typesetting so typing two spaces after a sentence are no longer needed yet old typewriter habits still continue. In proportional typesetting skinny letters are given less space than wide ones, white space between letters takes on a balanced appearance. Without the extra white space characteristic in mono-spaced type, breaks between sentences stand out in proportional typesetting without the need for two spaces after a line. Here are some articles with more information on the issue of one or two spaces after a sentence: