If A is a minimal algebra (that is, has no proper subalgebras) then the set S2(A) of all subalgebras of A2 has a natural structure of ordered involutive monoid. This paper gives a characterization of monoids S that appear in the role of this monoid if A is finite, weakly diagonal (every subalgebra of A2 contains the graph of an automorphism of A) and has a majority term.