Move the points by dragging them. Reload the page to restore the
initial configuration. There are many proofs known, but one of
the most attractive is due to the nineteenth century Belgian
mathematician Germinal Dandelin. It has the virtue of being
perhaps the most geometric in flavour. This proof has much in
common with the best known proof of Desargues' Theorem, in that
it can be interpreted in terms of a three-dimensional
configuration. Both Desargues' Theorem and Pascal's Theorem
assert that three points defined as the intersections of certain
pairs of lines in a configuration are collinear. One very
natural way to see that three points in a plane are collinear is
to find a plane other than the given one containing the three
given points, which must all therefore lie on the intersection
of two planes. For Desargues' Theorem, this plane is simple to
construct, for Pascal's not quite so simple. The three-
dimensional proof of Pascal's Theorem is now well known, but
seems to have originated with Dandelin. H.