[Editor's note: This article is part of our series of articles on installing and upgrading to Lion.]
As I explained in Installing Lion: What you need to know, one of the requirements for installing Lion is that you already have Snow Leopard (Mac OS X 10.6) version 10.6.6 or later installed. The main practical reason for this requirement is that Lion is available only via the Mac App Store, and the Mac App Store debuted in Mac OS X 10.6.6. In other words, you need Snow Leopard just to purchase and download Lion.
But once you've got your copy of Lion, can you install it onto a Mac or a hard drive containing Leopard (Mac OS X 10.5)?
The software license you agree to when you install Lion states that you can "download, install, use and run for personal, non-commercial use, one (1) copy of [Lion] directly on each Apple-branded computer running Mac OS X Snow Leopard or Mac OS X Snow Leopard Server...that you own or control." In other words, if your Mac shipped with Snow Leopard, you can install Lion on it. If your Mac shipped with Leopard, but you later purchased Snow Leopard for, and installed it on, that Mac, you can install Lion on it. If you didn't purchase Snow Leopard, you can't install Lion.
Those situations are pretty clear. But what if, for example, you've got a family-pack license for Snow Leopard, and you've got a Mac that shipped with Leopard but that you never upgraded to Snow Leopard? Assuming that Mac is compatible, the Lion license agreement says you can't upgrade to Lion until you first install Snow Leopard.
(This is just one scenario--I can think of a number of situations in which you might have Leopard on a Mac or an external hard drive, along with a valid license for Snow Leopard, and you'd rather not take the interim step of installing Snow Leopard just to upgrade to Lion. Having performed this two-step upgrade many times while researching our various Lion-installation articles, I can tell you that it's a real hassle.)
But lets take a step back. While the letter of the law says you need to install Snow Leopard before installing Lion, the spirit of the law seems to be that a particular Mac just needs a license for Snow Leopard before you can install Lion on it. In other words, in our view, you should be well within your rights to install Lion on any of your computers for which you have a valid, current Snow Leopard license--even if you don't install Snow Leopard on it first.
So then the question becomes whether there are any technical reasons you can't install Lion over Leopard. Based on our testing, the Lion installer refuses to install Lion onto a drive containing Leopard (10.5); in fact, it refuses to install on any drive running a version of Mac OS X below 10.6.6. It will, however, install onto a blank drive, so Lion clearly doesn't need any of Snow Leopard's files or settings.
You may be thinking, "It will install onto a blank drive? Then I'll just copy the installer to my Leopard-equipped Mac, connect an empty hard drive, install Lion there, and then use Migration Assistant to move my files over to it." Alas, while the Lion installer will freely install Lion onto a blank drive, the installer itself must be run from within Snow Leopard or Lion.
So how can you install Lion over Leopard? There are three ways: the official way, the brute-force method, and the quick-but-techie way.
The official way
As I explained above, Apple's official policy is that if you want to install Lion onto a Mac or a hard drive containing Leopard--assuming, of course, the Mac in question meets Lion's system requirements--you must first install Snow Leopard and then install Lion. This works, it's fairly easy to do--if a bit time-consuming--and it gets the Apple seal of approval.
The brute-force method
What if you don't want to install Snow Leopard first, or if you don't have your Snow Leopard disc handy? (I'm not being coy here--perhaps you've misplaced it, or maybe you're on the road and you've got your Mac's original [Leopard] disc with you as an emergency boot disc, but you don't have your Snow Leopard upgrade disc.)