I have been using one debian machine to run one program, without a desktop or windows, now I want to run two programs, and want to install Tabs Window Manager (TWM) to that end. I installed TWM in the usual way, successfully, as far as I can tell, but could use a hint on how to initiate it.

Likely need to run startx from your terminal - by the way, you don't need to use a GUI to run two programs, you can initiate a second terminal by using ctrl+alt+F2, or install screen or tmux multiplexers.

If you install a GUI (xfce4, or Gnome, for example) it will normally autoboot into the GUI. But, after the install, bring up a terminal, su to root, then you can issue one of the following one time commands.

systemctl get-default This will tell you what your default boot is. Probably graphical.target.

But, if you issue the following command...

systemctl set-default multi-user.target

...you will boot to a commandline only. From there you can issue startx (may need to install xorg) and bring up your windowing system. As fatmac said, to get to another commandline even while the GUI is running, use Ctrl-ALT F2, or F3, ... F6. To get back to your GUI, CTRL-ALT F1.

systemctl set-default graphical.target This will put you back to the default. i.e. boot into the GUI

I have not yet tried your suggestions but this is just to let you know that when I entered startx, I got nothing. I forget what the response message was exactly. More later..

Installing xorg did the trick! Many thanks.

First, I created .xinitrc in my user home directory.
Then startx worked, but only could open an xterm.
Then I installed xorg, and startx also executed twm.