http://www.captainsquartersblog.com/mt/archives/002483.php#c15399
IBM Executive (proportional spacing)
http://www.well.com/user/smalin/typinwhy.htm
excerpt:
"The IBM Executive typewriter I found at a garage sale was magnificent, and (having been long since replaced by the Selectric), dirt cheap. Only somebody with a PhD in secretarial skills could operate it. It was a proportional spacing machine: an 'm' was five spaces wide, an 'i' was two. There were two separate space bars (two and three spaces respectively). To correct a mistake, you had to know the width of all the characters involved so that you could backspace the appropriate amount (backspace was the only single-space key on the machine). There was an arcane procedure for producing justified type which involved typing a page a first time (while using a special guide to measure where the lines ended), noting the extra spaces that needed to be added, marking the copy to show where two-width spaces would be replaced with three-width spaces (or, in the worst case, two two-width spaces), and typing the page a second time. Even loading the ribbon (it was one of the first carbon ribbon machines on the market) was a major challenge: its rimless reels would spill their contents at the slightest mishandling, and the thin (less than 1/2" wide) tape had to be threaded through bewildering series of slots, grooves, carriers, and guides. It was a machine only a fanatic could love, and I did. I made regular trips to Santa Barbara's IBM parts center, and spent hours with tweezers, probes, hooks, needle-nosed pliers and other fine tools, getting it working right."
Just the kind of thing to whip off a memo with ... NOT!