Readers routinely ask me whether it’s still possible to clean install Windows 10 with an unused Windows 7 or Windows 8.1 product key. Over two years after Microsoft first enabled this functionality, the answer is a resounding yes.

And I can think of a number of reasons why one might need to do so.

As noted, Microsoft first introduced this capability over two years ago, and in doing so it erased what had been one of the early install/activation issues with the then-new Windows 10.

Almost a year later, I reported that this capability—which was supposed to be temporary, by the way–still worked. Since then, I’ve tested this scenario on a very regular basis, probably roughly once a month. And as people have asked me about it, on Twitter or via email, I’ve told them that it still works.

But it’s been a while since I’ve written on this topic formally. So here goes.

It still works.

What this means is that you can download the Windows 10 Setup media—which is always the latest version, so you’ll get Windows 10 version 1709, or the Fall Creators Update, at the time of this writing—and perform a clean install of the OS on any PC. And then you can activate that install of Windows 10 using an unused retail Windows 7, Windows 8, or Windows 8.1 product key. And it will just work.

You may think that most people will never need to do this. If your PC was already running Windows 7, 8, 8.1, or any version of Windows 10, a clean install of Windows 10 today will probably activate automatically anyway.

And that’s fair. A better way to look at this is that most people simply won’t be able to do this anyway. I mean, who has a bunch of unused retail Windows product keys hanging around anyways?

Some might. And if you have or have had an MSDN or TechNet subscription, all those old product keys will work too.

So let’s think about the scenarios where this might be useful.

It’s rare, but you might have a newly-built or purchased PC that did not come with any version of Windows.

You might want to clean install Windows 10 in a virtual machine (VM).

You might want to clean install Windows 10 on a Mac, either in Boot Camp or virtually.

You might want to upgrade from Windows 10 to Windows 10 Pro. Assuming you have the right kind of Windows 7, 8, 8.1 product key, this will work too. (See below.)

The unusual nature of most of these scenarios is what I think explains why this functionality is still working even though it was supposed to be temporary. It doesn’t hurt anyone. And if you really do need to do this, it’s nice to have.

That said, there are some important caveats.

That old Windows product key can only activate against an equivalent Windows 10 product edition. For example, a product key for Windows 7 Starter, Home Basic, and Home Premium can be used to activate Windows 10. And Windows 7 Professional and Ultimate product keys can activate Windows 10 Pro. It has to be a retail key, not a key that came with a computer. And it has to be unused, though there is some anecdotal evidence that even used keys will work in some cases. (And you could always call Microsoft support, explain the situation, and try for a phone activation.)

And here’s a fun future use for this feature. If you purchase a Windows 10 S-based and do not upgrade to Windows 10 Pro before the free upgrade offer ends next year, you can use a valid Windows 7, 8, 8.1 to do so. Yes. I’ve tried that too.

Anyway, you can activate Windows 10 at any time by navigating to Settings > Update & security > Activation. If it’s not activated, or if you simply want to upgrade from Windows 10 to Windows 10 Pro, you can do so from there.