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<p>objects contained by a line or a plane, such as <a href="page.php?w=line_segment">line segment</a>s, <a href="page.php?w=plane_region">plane region</a>s, <a href="page.php?w=vector_%28mathematics_and_physics%29">vectors</a>, <a href="page.php?w=direction_%28geometry%29">directions</a>, etc.</p>

<p>A <a href="page.php?w=surface">surface</a> is horizontal if its <a href="page.php?w=tangent_plane">tangent plane</a>s are everywhere perpendicular to the gravity vector at the tangent point or, equivalently, if the surface <a href="page.php?w=normal_vector">normal vector</a></p><p>
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