Where are my polygons?

I did a terrain plane using squares polygons (glBegin(GL_QUADS)). But naturally some polygons are only partially visible (intercept the frustum). The problems is these polygons are entirely invisible - if only a fraction is invisible, all the polygon are clipped. Why? What i am doing wrong?


Maybe the polys were drawn behind your view point or the polys are facing away from you or the color of the polys are black and your background is black as well.