How to change default resolution of X from the command line

Some times, while installation, or during working, a user may accidently change the default resolution of X window. In most cases, works OK, but what if the new video mode is not supported by the monitor? It can lead to any thing from malfunctioning to damaging of monitor. Due to this, it is not possible to run a configuration tool based on X. Because, on running X, your monitor my start blinking, or many colours may get spilled all over the monitior, or simple display may go. In this case, it is necessary to have a method to configure the X from command mode itself, such that resolution can be changed.
This can be done by editing a configuration file/etc/X11/X86Configuration.
We will have to issue the following commands:

1)su
2)On issuing this command, computer askes for super user password. Enter the super user password here. A super user terminal will be displayed.
3)cd/etc/X11
4)cp X86Configuration X86Configuration.old
5)vim X86Configuration
6)This will open the X86Configuration file for editing. Search for the following line:

Section Screen
7)In the SubSection Display, there is a line starting with Modes. This is the place where alterations should be made.
8)As an example, the line may look as:

Modes 800×600 640×480
This means that, mode 800×600 will be first tried, and on detacting any failure, 640×480 mode will be tried.

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