When drawing a graph, the programmer usually wishes to specify the coordinates of the points to be plotted in terms of the values of the variables involved. These coordinates are called world coordinates, and may have any floating-point value representable by the computer. The window refers to the rectangular region of world-coordinate space which is to be graphed. This window is mapped onto a rectangular region of the view surface, which is (a portion) of the screen or sheet of paper in the output device. This physical region onto which the window is mapped is called the viewport. Before a graph can be drawn, the program must define both the window and the viewport by calling appropriate routines in PLPLOT.