  Diophantine quadruples are sets of four distinct positive integers such that
the product of any two is one less than a square. All known examples belong to
an infinite set which can be constructed recursively. Some observations on
these regular solutions are presented. In particular we see how factors of the
four numbers satisfy relations which generalise the Stern-Brocot Tree. There is
also an update on the search for rational Diophantine sextuples with five new
examples.
