You may ask: If I use the MM_LOENGLISH mapping mode, will I start getting WM_SIZE messages in terms of hundredths of an inch? Absolutely not. Windows continues to use device coordinates for all messages (such as WM_MOVE, WM_SIZE, and WM_MOUSEMOVE), for all non-GDI functions, and even for some GDI functions. Think of it this way: The mapping mode is an attribute of the device context, so the only time the mapping mode comes into play is when you use GDI functions that require a handle to the device context as one of the parameters. GetSystemMetrics is not a GDI function, so it will continue to return sizes in terms of device units, which are pixels. And although GetDeviceCaps is a GDI function that requires a handle to a device context, Windows continues to return device units for the HORZRES and VERTRES indexes, because one of the purposes of this function is to provide a program with the size of the device in pixels.
However, the values in the TEXTMETRIC structure that you obtain from the GetTextMetrics call are in terms of logical units. If the mapping mode is MM_LOENGLISH at the time the call is made, GetTextMetrics provides character widths and heights in terms of hundredths of an inch. When you call GetTextMetrics for information about the height and width of characters, the mapping mode should be set to the same mapping mode that you'll be using when you draw text based on these sizes. As I cover the various GDI functions in this and subsequent chapters, I'll note whether they use device coordinates or logical coordinates.