A few days ago Larry Tesler passed. You may have seen news stories or the name may mean nothing to you. If you saw the news it probably mentioned that he was the guy behind Cut / Copy / Paste. This is true, he worked on the first graphical word processor while at Xerox PARC. In hindsight, it seems obvious now that it was needed, but like all things that seem obvious, they weren't until somebody came up with them.
What is less obvious and gets less press copy is his other major concept that he spent his life promoting, modeless user interfaces":
The word modeless to mean a user interface in which the user is never “stuck” in a mode (ca. 1970).
— Larry Tesler
Anybody who has ever used a non-graphical computer text editor - be it vi, vim, emacs, teco, wordstar, sed, sos etc - will understand what he means. You are either in "insert mode" or "command mode". When building the Gypsy Word Processor at PARC he wanted it to be modeless. The UI should not have "modes", you are either editing or editing. A graphical UI allowed this to happen.
Yet a lot of user interfaces today still have modes, especially on mobile devices. You want to reorder a list of items, you can't just drag them, you have to switch into edit mode, drag them around and click done. This is bad. You should be able to drag them around without this. Some applications get this right, many don't.
Early on as a software engineer, I read one of his papers from PARC about this and to this day if possible I avoid modes completely in all software that I build - I have a good record on this. If nothing else it makes it a lot easier for the user to use and understand.
I was at an event celebrating the 30th anniversary of the launch of the Macintosh, held at the same theatre that was used in 1984. Tesler was one of the guest speakers and during the interval, I managed to grab his attention very briefly to thank him for all the work that he had done and was continuing to do promoting modeless interfaces. He didn't know me and seemed bemused but looking back, I'm glad that I did.