From The Daily Mail:
Typing a double space after a full stop is wrong, period. With one space.
Microsoft Word is to start flagging the use of two spaces between sentences as an error — reflecting the standard accepted by most style guides.
The formatting error will begin being flagged by the popular word processing application with a blue wavy line.
Popular use of double-spacing is a hangover from the days of typewriting, when the equal-width characters of ‘monospaced’ fonts called for clearer sentence endings.
The introduction of proportional-spacing typewriters in 1944, however, began the process of rendering the extra space unnecessary for ensuring easy readability.
Nevertheless, the tradition of double-spacing continued — and is often found among those individuals who were first taught to type on a typewriter.
The new rule is being rolled out slowly across Word, meaning that users may not encounter the new warning until they update their software.
For the militant two-spacers among us, however, fear not — it will be possible to instruct the popular word-processing software to ignore the error.
Link to the rest at The Daily Mail
On an ancient history note, PG will add that proportional-spacing typewriters AKA the IBM Executive Typewriter were hated by secretaries.
(Yes, little Johnnie, there was a time when people called secretaries or typists were hired just for their keyboarding skills. And it pains me to say that those skills were much better than typing ability of anybody who grew up using a computer keyboard.)
The proportional spacing of the Executive typewriters made correcting errors hellish because if you hit the i key instead of the e key, there wasn’t enough space between the character that came right before the i and right after the i to white-out the i and squeeze in an e. So you had to retype the whole page.
At PG’s first job after graduating from college (where there was only one computer terminal in the department with a keyboard and printer that were housed in a separate room with a closed door because the printer made a tremendous amount of noise), only the secretary of the big boss had an Executive typewriter. Everybody else had monospace Selectrics.
PG recalls that the “Executive” secretary qualified for the machine by typing five pages of dictation without making a single error.