What key combination is used to forcibly halt the X server in the event of the graphical session becoming unusable (eg., desktop manager or fullscreen application hanging)?

`Ctrl+Alt+Del` is typically used to initiate a system reboot or shutdown, not to specifically halt the X server.

The key combination `Ctrl+Alt+Backspace` is a traditional X server shortcut designed to forcibly terminate the X server and restart the graphical login manager, providing a way to recover from a frozen or unresponsive graphical session. This shortcut is often disabled by default in modern distributions but remains a common knowledge point for troubleshooting X.

Pressing `Ctrl+Alt+Del` twice is also associated with reboot/shutdown behavior, not a direct X server termination.

`Ctrl+Alt+F1` (or other F-keys) is used to switch to a virtual console, which allows interaction with the command line but does not directly halt the X server; you would then need to manually stop the display manager.