What happened is that Ken Thompson came to Berkeley and brought this broken Pascal system, and we got this summer job to fix it. While we were fixing it, we got frustrated with the editor we were using which was named ed. ed is certainly frustrating.
We got this code from a guy named George Coulouris at University College in London* called em - Editor for Mortals - since only immortals could use ed to do anything. By the way, before that summer, we could only type in uppercase. That summer we got lowercase ROMs for our terminals. It was really exciting to finally use lowercase.
So we modified em and created en. I don't know if there was an eo or an ep but finally there was ex. [laughter] I remember en but I don't know how it got to ex. So I had a terminal at home and a 300 baud modem so the cursor could move around and I just stayed up all night for a few months and wrote vi.Bill Joy's greatest gift to man – the vi editor • The Register
ed→em→en→exという流れ。emのネーミング(Editors for Mortals)もいいな。
By the way, 'em' stands for 'editor for mortals' - I christened it that after Ken Thompson visited our lab at QMC while I was developing it and said something like: "yeah, I've seen editors like that, but I don't feel a need for them, I don't want to see the state of the file when I'm editing".George Coulouris: Bits of History
ところで、'em'は普通の人間のためのエディタ(editor for mortals)を意味する。私がemを開発している時に、Ken ThompsonがQMCの我々のラボを訪れこんなことを言ったため、私がそう名付けた。「ああ、こんな感じのエディタは見たことがあるよ。でも必要性を感じないんだ。私は編集中のファイルの状態を見たいとは思わないから」