Macs before and including the 17 inch flat panel iMac (about mid-2002) could boot up under OS 9, and Macs including and after the Power Mac G3 (blue & white) (early 1999) could run Mac OS X, and can probably be upgraded to 10.4.11. On Macs using Power PC processors and using Mac OS X through 10.4.11, OS 9 applications could be run in "Classic" mode on an installation of OS 9. So OS 9 could be there either to boot from or to support the Classic environment. In Mac OS X, you use System Preferences to select the startup disk in the Startup Disk pref pane, or Classic pref pane to setup and start the Classic environment.

You may find the Mactracker application at to be helpful in determining the capabilities of your particular Mac model.
Who serves his fellows, is of his fellows greatest - Chingachgook (by legend)