This happens mainly during updating a new version or maybe we remember during our work to shorten a command.
To shorten a command the acad.pgp file is used, actually commands belonged to it and abbreviations assigned to it. This is a text file, and you can edit it with a Notepad as well. It is not easy to find it tough, because during installation two versions are made, and this file is taken very deep in the library’s forest.
The first solution is to find and edit acad.pgp file with a line text editor. After it has been saved the program has to load it again. It generally happens when you restart AutoCAD.
There is a second solution however. It is easier to start Edit Program Parameters (acad.pgp) long named command of Customization command from the CAD Manager drop-down menu.
In this case you can edit the acad.pgp file immediately. You can find the same at AutoCAD 2010 at the end of the Ribbon menu Manage panel Customization group.
After customizing the commands, you should restart your computer to load the commands again, but with a new system variable, you can use the new commands immediately.
Write RE-INIT characters at command line, followed by 16 as a new value.
Having this done, you can use the new commands.