关于MM_LOMETRIC的疑问?

int nMode=MM_LOMETRIC;
switch (message)
{
case WM_CREATE:
return (0);

case WM_PAINT:
hdc = BeginPaint (hwnd, &ps);
SetMapMode(hdc,nMode);
hBrush=(HBRUSH)GetStockObject(GRAY_BRUSH);
hPen=(HPEN)GetStockObject(BLACK_PEN);
SelectObject(hdc,hPen);
SelectObject(hdc,hBrush);
Rectangle(hdc,0,0,500,-500);
EndPaint (hwnd, &ps);
return (0);
... ^
|
Rectangle（）里的坐标不是先左上角，后右下角吗，MM_LOMETRIC的坐标不是|-- >吗？那么（0，0）和（500，-500）怎么理解呢？为什么是-500呢？？