Unlike earlier versions of Windows, Windows 10 is regularly updated with new features based on user feedback, including before it was first released. Microsoft releases updates every half a year, an example of the software as a service principle. Each release has a four-digit build number (a type of version number), the first two digits referring to the year of release, and the other two digits referring to the month of release (e.g. "1903" refers to a build released in March 2019).