This message is sent when the size of a window's client area needs to be calculated.
| Parameter | Description |
| wParam | Is not used. | |
| lParam | Points to a RECT data structure that contains the screen coordinates of the window rectangle (including client area, borders, caption, scroll bars, and so on). |
The DefWindowProc function calculates the size of the client area, based on the window characteristics (presence of scroll bars, menu, and so on), and places the result in the RECT data structure.