OpenGl绘制的三维线段不显示？

我想在画茶壶的4个视区加边框，但是线段不显示？
glBegin(GL_LINES);
glVertex3f(-10.0f,0.0f,0.0f);
glVertex3f(10.0f,0.0f,0.0f);
glEnd();
glFlush();