I received a reply on Twitter from Bill Buxton, who worked at Xerox/PARC and other cool places, pointing me at this chapter out of his book: http://www.billbux

I received a reply on Twitter from Bill Buxton, who worked at Xerox/PARC and other cool places, pointing me at this chapter out of his book: http://www.billbuxton.com/input06.ChordKeyboards.pdf http://www.billbuxton.com/input06.ChordKeyboards.pdf The section "The NLS System: Engelbart and English" contains the most detailed explanation of Engelbart's chording keyboard that I've found, and the rest of the chapter is an interesting read.