This message occurs when the user presses the left mouse button.
| wParam | Contains a value that indicates whether various virtual keys are down. It can be any combination of the following values: |  | 
|  | Value | Meaning | 
|  | MK_CONTROL | Set if CONTROL key is down. | 
|  | MK_MBUTTON | Set if middle button is down. | 
|  | MK_RBUTTON | Set if right button is down. | 
|  | MK_SHIFT | Set if SHIFT key is down. | 
| lParam | Contains the x- and y-coordinates of the cursor. The x-coordinate is in the low-order word; the y-coordinate is in the high-order word. These coordinates are always relative to the upper-left corner of the window. |  |