With Windows 8 and now Windows 8.1, Microsoft tried - not entirely successfully - to make tablets part of a continuum that goes from number-crunching workstations and high-end gaming rigs through all-in-one touchscreen media systems and thin-and light notebooks down to slender touch tablets.
The general consensus is that it still has a long way to go to produce a unified OS.
Despite rumours of an aggressive development and shipping schedule, there's no official word about what's in the next version of Windows, but there are plenty of rumours (many of them from Chinese enthusiast sites that claim to have leaked builds), plus more reliable information from job adverts for the Windows and Windows Phone teams.
There are also patents, which may or may not be relevant, and some rare comments from developers on the Windows team. Here's what we've heard about Windows 9 and what we think is happening.
What is it? A complete update of Windows
When is it out? We expect it to be out in 2015
What will it cost? We really have no idea. But if Windows 8 is anything to go by, it won't cost much to upgrade.
Windows Blue turned out to be Windows 8.1 rather than a completely new version of the Windows OS - Windows 9 will be that new version.
As for interim releases, we'll probably also get Windows 8.2 before we get Windows 9. And we know that we're definitely going to get an initial update to Windows 8.1 - called Windows 8.1 Update 1 after a previous goof already let it out of the bag.
The new update features improvements to the Start Screen including the ability to boot straight into the Desktop, the return of shutdown on Start and a more familiar taskbar to unify the old and new user interfaces.
It will be announced at Build 2014.
It certainly seems there's a new development cadence for Windows in action. It seems that Microsoft is set to put out new releases of Windows, Windows RT and Windows Server every year, the way it already does for Windows Phone.
The next complete version of Windows is being referred to as Windows 9, though this may change. And a new codename has appeared - Threshold, possibly in refrence to moving across from our reliance on the desktop to a new world where the Start screen is at the heart of how we use Windows.
While still just a codename, Windows 9 was referenced by Microsoft in a job posting, spotted by MSFT Kitchen on 13 March 2013.
The ad, for a Bing Software Development Engineer, says that the team will be delivering products "in areas including Windows 9, IE11 services integration, touch friendly devices including iPad and more."
Windows 9 release date
Microsoft communications chief Frank Shaw said the company wasn't ready to talk about how often Windows might come out when we spoke to him in January, but he agreed "you have certainly seen across a variety of our products a cadence that looks like that; Windows Phone is a good for example of that, our services are a good example of that".
We don't know if Windows 9 will be available as an upgrade from Windows 7 that you can buy as a standalone product or if you'll have to have Windows 8 to get the upgrade. But it may not be with us for a while yet - Windows business chief Tami Reller has talked about "multiple selling seasons" for Windows 8, meaning that we'll likely have several versions of it.
In January 2014, well-known Microsoft blogger Paul Thurrott said he believes the company plans to release Windows 9 (codenamed Threshold) in April 2015, less than three years after Windows 8.
The thinking appears to be that the Windows 8 name is now too tarnished and that - in contrast to Reller's comments above - Microsoft wishes to clear things out by releasing Windows 9 instead.