Lines through a point of a smooth cubic surface
= Lines through a point of a smooth cubic surface
{title2=$\#\{L\subset S:P\in L\}\le3$}
Every <projective line> through $P$ on a <smooth cubic surface> lies in its <projective tangent plane>. Each is a distinct linear factor of the cubic <plane section>. There can therefore be at most three. Equality occurs when the tangent section consists of three concurrent lines.