P.I.T.E. - Spaces After Period: "Everyone who took typing class on real typewriters has been taught to use two spaces after a sentence-ending period. As I understand it, this is an artifact unique to typewriters: because the technology results in monospaced text (as others have pointed out), single spaces don't give a strong visual cue; also, because typewriters were typically used in business writing, where abbreviations (terminated with periods) are more common, the convention was adopted to help people distinguish between sentence-ending periods and abbreviation-ending periods.
I learned, however, that one reason typesetters avoid double-spacing after periods is also rooted in the technology of the Linotype machine. Linos use wedges for spaces, and rectangular slugs for type. After a line has been set, a handle is pulled to force the wedges up and justify the line. Two adjacent wedges would jam up the works, so if a customer insisted on two spaces, blank rectangular slugs would need to be inserted, making more work for the Lino operator."
