From David McKenzie’s blog <https://blog.djmnet.org/2008/08/05/origin-of-emacs/>:

EMACS was written in TECO, but not the TECO that most people know.
Most people know a very stripped-down and bastardized TECO which
Bob Clements adapted for the DEC operating system for the PDP-6.

...

The original TECO used a display scope to display the text being
edited around the editing point. It remained a source of wonder
and astonishment to the original authors of TECO that anyone ever
used it without a display screen.

...

This guaranteed EMACS a place in the sun. Without McMahon and
Stallman's efforts we'd probably all be using vi or worse today.

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