WindowMargins->{{left, right}, {bottom, top}} specifies the distances from each edge of your main display screen to each edge of the notebook window.
Typically only two distances are given explicitly; the others are Automatic, indicating that they should be determined from the size of the window.
Explicit distances are given in printer's points.
Negative values represent edges that are off your main display monitor, but may be visible on additional monitors.
The settings for WindowMargins change whenever you move a window around interactively using the front end.
Window edges closer to the edges of the display screen are typically assigned explicit margin distances; the others are set to Automatic. This allows the same setting for WindowMargins to work on screens of different sizes.
With the default setting WindowMargins->Automatic, new windows are placed on the screen in such a way as to make as many window title bars visible as possible.
EXAMPLES
Basic Examples 
Scope 