  In an earlier paper by three of the present authors and Csaba Schneider, it
was shown that, for $m\ge2$, a set of $m+1$ partitions of a set $\Omega$, any
$m$ of which are the minimal non-trivial elements of a Cartesian lattice,
either form a Latin square (if $m=2$), or generate a join-semilattice of
dimension $m$ associated with a diagonal group over a base group $G$.
  In this paper we investigate what happens if we have $m+r$ partitions with
$r\geq 2$, any $m$ of which are minimal elements of a Cartesian lattice. If
$m=2$, this is just a set of mutually orthogonal Latin squares. We consider the
case where all these squares are isotopic to Cayley tables of groups, and give
an example to show the groups need not be all isomorphic. For $m>2$, things are
more restricted. Any $m+1$ of the partitions generate a join-semilattice
admitting a diagonal group over a group $G$. It may be that the groups are all
isomorphic, though we cannot prove this. Under an extra hypothesis, we show
that $G$ must be abelian and must have three fixed-point-free automorphisms
whose product is the identity. Under this hypothesis, such a structure gives an
orthogonal array, and conversely in some cases.
  If the group is cyclic of prime order $p$, then the structure corresponds
exactly to an arc of cardinality $m+r$ in the $(m-1)$-dimensional projective
space over the field with $p$ elements, so all known results about arcs are
applicable. More generally, arcs over a finite field of order $q$ give examples
where $G$ is the elementary abelian group of order $q$. These examples can be
lifted to non-elementary abelian groups using $p$-adic techniques.
