Office 365 launched in 2011 as a subscription-based alternative to Microsoft's popular Office productivity suite. Powered by Microsoft's own Azure cloud platform, Office 365 allows users on Windows and OS X to make use of the full-fat office suite while cut-down versions are also available on other platforms via the web browser. The system also ties in to the OneDrive storage platform and Skype communications software, while its monthly subscription fee is considerably lower than buying Office outright - even when you don't get a year's free subscription as part of the purchase of a Windows 8.1-based device, a frequently-used sweetener.When Microsoft formally unveiled its launch plans for Windows 10 last month , the company made much of its plans to offer 'Windows as a Service.' While Something-as-a-Service normally suggests a subscription model similar to that of Office 365, at the time Microsoft's Joe Belfiore suggested that his company was using the phrase to mean an end to regular and full-price operating system releases in favour of a rolling-release methodology which should see continuous upgrades to the platform over the coming years.While the option of a subscription-based Windows wasn't discussed at the event, it certainly appears to be something Microsoft is actively investigating. The company has registered a trademark for 'Windows 365,' a name which suggests at least the same relationship between it and Windows as between Office 365 and Office.First spotted by Microsoft-watcher Neowin , the word mark filing - in the computer software, telecommunications services, electronic storage, training, and computer services categories - is recent, dating to the 29th of January. Coming as it did post-press-conference, the timing could suggest a defensive filing: Belfiore's comments on Windows as a Service led to numerous comments about the possibility of a subscription-based Windows licence option, and Windows 365 would be the obvious name for it. It may also suggest that the rumours regarding the impending launch of a subscription tier for the operating system are true, although if so it seems odd that Microsoft would have waited so long to register the trademark.Microsoft, naturally, has not commented on its plans for the trademark nor on whether it will release Windows 10 or any subsequent versions as a subscription service in the future.