The system menu in the upper-left corner of a standard frame window is different from the menus defined by the application. The system menu is controlled and defined almost exclusively by the system; your only decision about it is whether to include it when creating a frame window. (It is unusual for a frame window not to include a system menu.) The system menu generates WM_SYSCOMMAND messages instead of WM_COMMAND messages. Most applications simply use the default behavior for WM_SYSCOMMAND messages, although applications can add, delete, and change system-menu entries.