An early reference to a finite projective plane is in the paper by Veblen
[32], which studied the axioms for geometry and used the plane
of order 2 as an example. Veblen also proved that this plane of order 2 cannot
be drawn using only straight lines. In a series of papers
[32,33,34], Veblen, Bussey and Wedderburn
established the existence of most of the planes of small orders, as well
as all four non-isomorphic planes of order 9. One of the orders missing is
n = 6.