[Editor's note: This article is part of our series of articles on installing and upgrading to Mavericks.]
As I explained in my guide to installing Mavericks, one of the requirements for installing OS X 10.9 is that you already have at least Snow Leopard (Mac OS X 10.6) installed. (Specifically, Mavericks requires OS X 10.6.8 or later.) The main practical reason for this requirement is that Mavericks is available only via the Mac App Store, and the Mac App Store debuted in Mac OS X 10.6--in other words, you need Snow Leopard or later just to be able to purchase and download Mountain Lion.
But once you've got your copy of Mavericks, can you install it onto a Mac or an external drive containing Leopard (Mac OS X 10.5)?
The license agreement
The software license you agree to when you install Mavericks states that you can "download, install, use and run for personal, non-commercial use, one (1) copy of [the OS] directly on each Apple-branded computer running OS X Mountain Lion, OS X Lion or OS X Snow Leopard...that you own or control." In other words, if your Mac shipped with Mountain Lion, Lion, or Snow Leopard, you can install Mavericks. If your Mac shipped with Leopard or Tiger (Mac OS X 10.4), but you later purchased and installed Snow Leopard, Lion, or Mountain Lion, you can install Mavericks. If your Mac doesn't at least have Snow Leopard installed, you can't install Mavericks.
That seems 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's never been upgraded to Snow Leopard, Lion, or Mountain Lion? The Mavericks license agreements say that even if that Mac is compatible, you can't upgrade to 10.9 until you first install at least 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 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 Mavericks. Having performed this two-step upgrade many times while researching our various Mavericks-installation articles (and the past two years while writing our upgrade guides for Lion and Mountain Lion), I can tell you that it's a real hassle.
The practical question
But lets take a step back. While the letter of the law says that you need to install at least Snow Leopard before installing Mavericks, the spirit of the law seems to be that a particular Leopard-equipped Mac just needs a license for Snow Leopard, Lion, or Mountain Lion before you can upgrade it. In other words, in our view, you should be well within your rights to install Mavericks on any of your computers for which you have a valid, current Snow Leopard, Lion, or Mountain license--even if you don't actually install Snow Leopard first.
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