While built-in support for Classic is gone in 10.5:
A few Macs officially support both booting into 9 and 10.5 without an additional partition or hard drive with booting via Apple menu -> System Preferences -> Startup Disk in 10.5, and Apple menu -> Control Panels -> Startup Disk in 9 or booting holding the X key on a restart from Mac OS 9 boot.
PowerMac G4 867 and faster QuickSilver
PowerMac G4 1 through 1.25 Ghz MDD with no Firewire 800 port * :
eMac 1 Ghz model M8950LL/A
Powerbook G4 867 Mhz and greater with rear USB ports between the hinges under a flap.
No iMacs, nor iBooks meet the requirement.
Some CPU upgrade card Macs may support dual booting. Check with the third party manufacturer of the CPU upgrade card if that is possible.
While older Macs may work with Leopard with a special Target Disk Mode based install, this is not an officially recognized configuration, meaning any attempt to do so is on your own, with all the risks of maintaining and upgrading that configuration up to you.
Backup your data at least twice before attempting any upgrades.
It is possible to add an external firewire hard drive, or partition an existing hard drive of a Mac that supported Tiger to have a dual-boot Tiger/Leopard configuration on 867 Mhz G4 and faster PowerPC Macs. Partitioning will wipe the contents of the hard drive, so backup your data at least twice before proceeding. Here's how to partition:
This will also allow using Classic in the Tiger booting session if installed from the Mac's original restore discs:
and/or retail installer for 9 if newer than those discs.
Those discs can typically be obtained by calling AppleCare if you lost them, or asking the original seller of the machine to give you the restore discs. If they lost them, they should call AppleCare on your behalf and give them to you.
AppleCare can be reached here:
The ports on the back of the MDD PowerMac G4 image above show where Firewire 800 is, if it is present. When it is not present in that location, the MDD PowerMac G4 can boot into Mac OS 9, even when Mac OS X 10.5 is installed, via Apple menu -> System Preferences -> Startup Disc. When Firewire 800 is present you must install 10.4.11 or earlier on a second hard drive internally, a partition of the 10.5 or later drive, or an external Firewire hard drive if you wish to use Mac OS 9 in its Classic environment. Booting is not available to Mac OS 9 with Firewire 800 built-in PowerMac G4s. Either way, to get Mac OS 9 on the PowerMac G4 MDD models, you need to use the original PowerMac G4 installer discs that came with it following the restore directions below: