The sequence of three OpenGL function calls glMatrixMode(GL_PROJECTION), glLoadIdentity() and gluOrtho2D(0.0, 1.0, 0.0, 1.0) establish, in essence, the boundaries of our drawing surface. The four parameters in the call to gluOrtho2D are, respectively, the left, right, bottom and top boundaries. For our example, we are creating a drawing surface that is the unit square with the lower-left corner at (0.0, 0.0) and the upper-right corner at (1.0, 1.0). We more frequently call the drawing surface the projection window, which we study in more depth through the tutorial OrthographicProjection.