How to define from what partition to boot by default in OS X?

I have installed OS X 10.5, 10.6 and 10.7 on different partitions of my Mac (developer machine).

By default 10.7 is booted. Of course, I can change it at each boot time by holding the alt key, but how to, for example, change the default boot partition to the 10.6 partition?

Solution 1:

Is System Preferences → Startup Disk what you want?

The selection should be permanent. Also take a look at the Apple support page:

This [manual] startup volume selection is temporary. To set the default startup volume for your Mac after your computer is running, open System Preferences and choose Startup Disk from the View menu (in Mac OS X) or open the Boot Camp control panel (in Windows).

There seems to be a certain keyboard shortcut to make the selection permanent from the boot screen itself:

Boot your machine with the Option key down to invoke the Startup Manager, then hold down the Ctrl key prior to clicking on a volume to boot. (You may have to use the left and right arrow keys to select a volume first.)

If this trick is supported on your Mac, you’ll notice that upward-pointing arrow—the one that points to the selected disk—will change into a circular arrow instead.