glBegin(GL_LINE_LOOP) ;
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 0.5, 0.0);
glVertex3f(0.5, 0.5, 0.0);
glVertex3f(0.5, 0.0, 0.0);
glEnd() ;
glBegin(GL_LINE_LOOP) ;
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 0.5, 0.0);
glVertex3f(0.5, 0.5, 0.0);
glVertex3f(0.5, 0.0, 0.0);
glEnd() ;