AFAICT, in ttys you can send a `C-_` to Emacs either by pressing `C-/` (e.g. xterm, uxterm, rxvt, xfce4-terminal, gnome-terminal) or by pressing `C--` (e.g. rxvt, Linux console).
AFAICT, in ttys you can send a `C-_` to Emacs either by pressing `C-/` (e.g. xterm, uxterm, rxvt, xfce4-terminal, gnome-terminal) or by pressing `C--` (e.g. rxvt, Linux console).