always) visually contained within their parents. MSG (Message) Applications and the operating system communicate with HWNDs using messages. A window message is the combination of a target HWND, a message code, and other details such as mouse position. Message Loop By combining calls to Windows USER in the right way, a Windows application processes messages. The core Windows message loop (including calls to GetMessage, TranslateMessage and DispatchMessage) is at the core of any Windows application. Developer functionality related to Windows USER is provided in the C header file winuser.h. Implementation In 16-bit versions of Windows, Windows USER was implemented as a file called user.exe. The file extension here was a misnomer, as Windows USER was in fact always a dynamic link library. In 32-bit versions of Windows, the 32-bit version of Windows USER is called user32.dll and is located in the System32 directory, while compatibility with 16-bit applications is provided by having a modified version of user.exe also present. In 64-bit versions of Windows, the 64-bit implementation of Windows USER is called user32.dll and is located in the System32 directory, while a modified 32-bit version (also called user32.dll) is present in the SysWOW64 directory. See also Graphics Device Interface (GDI) Microsoft Windows library files References External links Comprehensive Reference on WOW64 Folders, Variables, Values and References Windows NT architecture Windows components Windows technology