Mac OS X 10.1: Cannot Install 10.1 Over 10.2 or While Started Up From 10.2

The Mac OS X 10.1 installer will not open on a computer that is started up from Mac OS X 10.2, or install on a volume that already has Mac OS X 10.2 installed on it

Symptom

The Mac OS X 10.1 installer does not open if the computer is started up with Mac OS X 10.2.
A Mac OS X 10.2 volume or partition is dimmed in the Mac OS X 10.1 installer.

Solution

To install Mac OS X 10.1 on a Mac OS X 10.2 volume or partition, start up from the Mac OS X 10.1 installation disc and erase the target volume.

Warning: Choosing Erase and Install completely erases the contents of the selected volume. Be sure to back up your files before continuing. To learn more, see technical document 106941, "Mac OS X: How to Back Up and Restore Your Files".