Microsoft today quietly put an end to sales of Windows 7 licenses to computer makers, marking a major milestone for the seven-year-old OS.

According to Microsoft's rules, the Redmond, Wash. company stopped selling Windows 7 Professional or any version of Windows 8.1 to OEMs (original equipment manufacturers) as of Oct. 31.

The end of Windows 7 and Windows 8.1 left only Windows 10 as a long-term choice for OEMs that pre-load Windows on their wares.

The original end-of-sales deadline for Windows 7 Professional was to be Oct. 31, 2014 -- two years after the launch of Windows 8 -- but early that year Microsoft broke with practice and only called for an end to consumer systems. It left open the cut-off for Windows 7 Professional, saying it would give a one-year warning before it demanded that OEMs stop selling PCs with that edition.

Microsoft issued that warning a year ago.

Organizations with enterprise licensing agreements and Software Assurance -- the annuity-like program that provides additional rights -- may continue to purchase new PCs, then downgrade the OS from the already-installed Windows 10 to Windows 7 if they want to keep using the older edition.

And new Windows 7 Professional PCs won't vanish immediately; OEMs will be allowed to use what licenses they have in stock.

For example, Dell's online store today still listed 17 different notebook configurations equipped with Windows 7 Professional. The same goes for smaller computer sellers, like Puget Systems, an Auburn, Wash. custom PC maker: Such shops can continue to build new Windows 7 Professional PCs until their supply of licenses dries up.

Microsoft pulled the plug on Windows 7 even though it remains the most popular operating system on the planet. Windows 7 has lost about a fifth of its user share since the mid-2015 launch of Windows 10, but according to U.S. analytics vendor Net Applications, it powered 48% of all personal computers in October, more than twice Windows 10's share.

Windows 7 support is to continue until January 2020, giving users just over three years to migrate to another operating system.