The X Window System uses four configuration files: 
- .Xdefaults
 Specified default appearance and behavior characteristics for clients. The contents of this file are covered in more detail in chapter 5.
- .x11start
 Specifies the clients that start when the X Window System starts.  This file is covered in more detail in chapter 4.
- .mwmrc
 Specifies the menus, menu selections, button bindings, and keyboard bindings that control the OSF/Motif Window Manager (mwm).  The contents of this file are discussed in chapters 5 and 7
- app-defaults/*
 Optional configuration for specific clients.  The contents of this file are discussed in chapter 5.
If your home directory does not contain these files, the X Window System uses the system-wide versions of these files in /usr/lib/X11.
If you want to customize your X environment, copy these files from the 
/usr/lib/X11 to your home directory (noting the name change), and make your modifications.  For example: 
   cp /usr/lib/X11/system.mwmrc $HOME/.mwmrc
   cp /usr/lib/X11/sys.Xdefaults $HOME/.Xdefaults
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The X server looks first in your home directory for these files.  If they are not there, it uses the system-wide files.