Microsoft has confirmed that Windows 10 will be offered as a free download to customers using Windows 8.1, Windows 7 and Windows Phone 8.1.
Announced during its Windows 10 media briefing, the free upgrade will be available to all of those customers - 200 million using Windows 8.1 alone - for an entire year after its official release date.
At present, that release is yet to be revealed. It is believed though that Microsoft will make the consumer version available for download from mid-2015 - summertime.
The revelation that Windows 10 will also be available as a free upgrade for Windows Phone users means that the company has confirmed it is killing off the separate Windows Phone operating system. Instead, Windows 10 is scaleable and will form the system software of all the devices under Microsoft's banner. That includes laptops, tablets and desktop computers, naturally, but also Xbox One, smartphones and wearables.
By ensuring that the operating system is unified, developers can develop for one platform, knowing that their software will work on multiple devices. This will give them an extraordinary reach. It also means that Windows Phone owners won't have to worry about more app being available on Android and iPhone as the platform will encourage developers to include Windows in their plans.
Microsoft has confirmed that it will "offer a free upgrade to Windows 10 for qualified new or existing Windows 7, Windows 8.1 and Windows Phone 8.1 devices that upgrade in the first year."
Once devices are upgraded they will "continue to keep it up to date for the supported lifetime of the device, keeping it more secure, and introducing new features and functionality over time – for no additional charge."
However there are some exclusions:
"Devices must be connected to the internet and have Windows Update enabled. ISP fees may apply. Windows 7 SP1 and Windows 8.1 Update required. Some editions are excluded: Windows 7 Enterprise, Windows 8/8.1 Enterprise, and Windows RT/RT 8.1."