System variables are settings that AutoCAD checks before it decides how to do something. For example, if you set the system variable SAVETIME to 10, AutoCAD automatically saves your drawing file every ten minutes; if you set SAVETIME to 60, the time between automatic saves is one hour. Hundreds of system variables control AutoCAD's operations.
To change the value of a system variable, just type its name at the AutoCAD command prompt and press Enter. AutoCAD displays the current value of the system variable setting and prompts for a new value. Press Enter to confirm the existing setting, or type a value and press Enter to change the setting.
Being able to change system variables by typing their names is a boon to power users and occasionally a necessity for everybody else. The only problem is finding or remembering what the names are. In most cases, you'll be told what system variable name you need to type — by me in this book or by the local AutoCAD guru in your office.
Here are the three kinds of system variables:
Those saved in the Windows Registry: If you change this kind of system variable, it affects all drawings when you open them on your system, but not necessarily on other computers.
Those saved in the drawing: If you change this kind of system variable, the change affects only the current drawing, but on any computer.
Those that aren't saved anywhere: If you change this kind of system variable, the change lasts only ...