Watermark

Watermark is a common name for a label on the bottom-right corner of the Windows desktop identifying the operating system version, which overlays the desktop wallpaper. It is most commonly associated with pre-release versions, where it is enabled by default with no option to disable it unless one modifies the system files. It is possible to enable it on release builds by modifying or adding a registry entry. A distinctive form of watermark is also visible when a Windows system boots in safe mode, overriding any other forms of watermark, if applicable. Debug builds will also display the path to the currently loaded Windows installation below the version information.

Watermarks originally used to be displayed on checked/debug builds only. Since the development of both Windows 98 and Windows 2000, they have become widely used to mark pre-release builds regardless of the build type.

History
The first Windows build known to display a kind of watermark is Windows 3.0 Debug Release 1.14; it was later removed in the lead up to the RTM. Watermarks were reintroduced during the development of Windows 3.1 as seen in the SDK debug kernel which, if applied on top of an existing copy of Windows, displays a shaded label in the bottom right corner identifying the current operating mode and the Windows version. The details of the current operating mode were removed during Windows 95 development, as they became redundant with the removal of Standard mode and 286 support.

Watermarks did not appear on non-debug builds until Windows 98 build 1351 and Windows 2000 build 1989.1. Enabling Active Desktop would remove the watermark until it was disabled, as the Active Desktop page overlays the classic desktop including the watermark.

Watermarks can sometimes include legal notices or warnings. The Aero demo compile of Windows Longhorn build 4050 contains copyright and trademark notices in the bottom left corner. Starting with Windows 8 build 7788, internal builds also display a separate message stating that the software is Microsoft confidential material, in addition to warning users about the legal consequences of sharing these builds outside of Microsoft; the same warnings were also shown in. The last known build to contain this warning message is Windows 10 build 14389.

In internal Windows 11 builds (such as build 21990), a message is displayed on the bottom right corner which simply reminds the user to not take any screenshots of the build that they are using. In Windows 11 build 22000.51 and above, an additional watermark may be automatically placed on the top right corner of the desktop if the current SKU is SE, stating that the machine is not set up securely and that potential data loss could occur.

Safe mode
When starting Windows 9x or NT (at least in Windows 2000 build 1738.1) in safe mode, the OS displays watermarks in every corner saying "Safe Mode". Additionally on Windows NT, the build number is displayed on the top of the desktop (similar to the debug compiles of Windows NT before 2000) instead of the bottom right.

PaintDesktopVersion
In Windows NT-based builds that do not show the watermark by default, it is possible to enable it through the  registry value:

Windows 98 and Me use a string value instead: