This article is from the Macintosh system software FAQ, by Elliotte Harold email@example.com with numerous contributions by others.
By far the best way is to divide your disk into multiple
partitions, one partition for each system folder. Then use your
formatting software to select the partition to boot from. This
will, however, trash everything on your hard disk so back up first.
Soft partitions like those created by Norton Utilities and other
utility packages are not nearly as reliable or safe for your data
as hard partitions created by a disk formatter like Drive7.
If you don't want to repartition your hard drive, you can keep
compressed archives of system folders you might want to use on your
hard disk. To switch system folders you'll need to boot off a
floppy or a second hard disk, compress the old system folder, and
uncompress the new one. Just be sure that when you boot your Mac
there's not more than one uncompressed System Folder on any one
Finally if you absolutely must keep multiple, bootable system
folders on the same hard disk, Keisuke Hara's freeware System
Switcher 1.1 or Kevin Aitken's System Picker 1.0.1 will adjust
the boot blocks of the hard disk so you can pick which one your
Mac will boot off from. See
If you put a copy in the Startup Items folder of your System 7
system folder, and specify it as a startup item in System 6,
then whenever you start up you'll be offered a choice of systems.