On the first day of its Build 2017 developer conference in Seattle, Microsoft announced that Windows 10 is now running on 500 million "monthly active devices."
(That metric includes devices that have been active in the past 28 days, Microsoft officials have said in the past. The figure includes not only Windows 10 installed on PCs, tablets, and phones, but also on Xbox One consoles and a very small number of HoloLens and Surface Hub devices.)
The half-billion milestone is an important one for convincing developers to write software for the Universal Windows Platform and to convert desktop apps so they can be sold in the Windows Store.
Ironically, though, that seemingly large number is also a slight disappointment. At the Build conference in 2015, Windows boss Terry Myerson set an audacious goal for Windows 10: It would be installed on 1 billion devices within two to three years, meaning by late summer 2018.
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A year later, after Microsoft sold off its Nokia subsidiary and threw in the towel on its Windows 10 ambitions, Myerson was forced to admit that getting to a billion Windows 10 devices would take longer than expected.
The biggest boost to Windows 10 adoption, of course, was the year-long free upgrade offer that began with the operating system's launch in July 2015.
The ticker climbed steadily throughout 2016, with Microsoft making frequent announcements of Windows 10's momentum: 200 million at the beginning of the year, 270 million at the end of March, 300 million as of May 5, and then 350 million in a report on June 29, 2016, just before release of Anniversary Update.
By the end of September 2016, roughly six months later, the number of monthly active devices running Windows 10 had increased to 400 million.
Today's milestone is the first such announcement in more than seven months, and the pace has slowed significantly.
Historical comparisons of these milestones are difficult.
Microsoft has never been shy when it comes to bragging about Windows momentum, but for earlier releases, the company typically reported the number of licenses sold rather than actual installations.
In the case of Windows 7, probably its most successful release ever, Microsoft said it had sold 350 million licenses after 18 months. But that total included some machines that were still on store shelves and in warehouses, and it didn't account for corporate users who bought those licenses on new PCs and then downgraded them to Windows XP.
The Windows 10 number doesn't answer the biggest question of all: Are enterprise customers making the upgrade?
A recent survey from Gartner predicts that 85 percent of organizations will start a Windows 10 production deployment by the end of 2017. A separate survey of IT pros from Dimension Research said that 77 percent of organizations surveyed say they will complete their Windows 10 migrations within the next two years.
There's a certain inevitability to the steady climb in the Windows 10 installed base. Windows 7, which still dominates corporate computing, will reach the end of its support lifecycle in less than three years, and Windows 10 is the logical successor.
At the current pace, Windows 10 will pass the billion-device mark right around the time that deadline expires in January 2020. The real question at that point is: How many PCs will still be running Windows 7?