System messages are sent by Windows to an application when a user accesses a window's System menu, scroll bars, or size box. Although an application can process these messages, most applications pass them on to the DefWindowProc function for default processing. The following list briefly describes each system message:
| Message | Description | |
| WM_SYSCHAR | Results when a WM_SYSKEYUP and a WM_SYSKEYDOWN message are translated. | |
| WM_SYSCOMMAND | Sent when the user selects a command from the System menu. | |
| WM_SYSDEADCHAR | Results when a WM_SYSKEYUP and a WM_SYSKEYDOWN message are translated. | |
| WM_SYSKEYDOWN | Sent when the user holds down the ALT key and then presses another key. | |
| Message | Description | |
| WM_SYSKEYUP | Sent when the user releases a key that was pressed while the ALT key was held down. |