The Emacs keybindings of "C-x [space]" described in a maybe still widely referred tutorial are outdated.

I've noticed the "An Introduction to GDB in Emacs" here [1].

The above introduction says the following:

```
Set a breakpoint by going to the source line of the program you're
debugging, and typing:

C-x [space]

(That's the space bar.) GDB will tell you that you've just set a breakpoint.
```

But I noticed the following key binding by running `C-h k C-x [space]
RET` on my machine:

```
rectangle-mark-mode is an autoloaded, interactive and compiled
function defined in rect.el.gz.

Signature
(rectangle-mark-mode &optional ARG)

Documentation
Toggle the region as rectangular.

This is a minor mode.  If called interactively, toggle the
Rectangle-Mark mode mode.  If the prefix argument is positive,
enable the mode, and if it is zero or negative, disable the mode.

If called from Lisp, toggle the mode if ARG is toggle.  Enable the
mode if ARG is nil, omitted, or is a positive number.  Disable the
mode if ARG is a negative number.

To check whether the minor mode is enabled in the current buffer,
evaluate rectangle-mark-mode.

The mode's hook is called both when the mode is enabled and when it is
disabled.

Activates the region if needed.  Only lasts until the region is deactivated.

View in manual

Key Bindings
ctl-x-map SPC
global-map C-x SPC
```

[1] https://courses.cs.washington.edu/courses/cse378/97au/help/gdb-emacs.html

Regards,
-- 
Assoc. Prof. Hongsheng Zhao <[email protected]>
Theory and Simulation of Materials
Hebei Vocational University of Technology and Engineering
No. 473, Quannan West Street, Xindu District, Xingtai, Hebei province