Watermark

Watermark is a common name for a label in 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 without tampering with system files, however, it is also possible to enable it on release builds with a registry entry. A distinctive form of watermark is also visible when a Windows system boots in safe mode, which overrides any other forms of watermark, if any.

Watermarks originally used to be displayed on checked/debug builds only. However, during the development of 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. However, this 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 when applied on top of an already installed copy of Windows displays a shaded label in the bottom right corner identifying the current operating mode and the Windows version. The operating mode part was removed during Windows 95 development, as it became redundant with the removal of Standard mode and 286 support.

Watermarks didn't appear on non-debug builds until Windows 98 build 1351 and Windows 2000 build 1989.1. However, enabling Active Desktop would remove the watermark until it was disabled. Starting with Windows 8 build 7788, internal builds also display a message stating that the software is Microsoft confidential material and warning users of the legal consequences of leaking these builds outside of Microsoft as a part of the watermark. A similar warning was also shown in. The last known build to contain this feature is Windows 10 build 10587.

PaintDesktopVersion
In builds of Windows that don't show the watermark by default, it is possible to toggle it on through the  registry value:

Windows 98 and ME use a string value instead: