DoubleCAD uses a multiple document interface (MDI). This means that the DoubleCAD application window can contain multiple windows, each of which contains a DoubleCAD drawing. Whenever you open a drawing or start a new drawing, the drawing will appear in its own window within the DoubleCAD application window. You can also use multiple windows to display varying views of the same drawing.
One advantage of the multiple document interface is that it allows you to intuitively move and copy objects from one drawing to another.
The window commands can be accessed from Window menu or from the Window toolbar, opened by right-clicking on any toolbar and selecting Window.