A variety of associative rings B
immediately covers its subvariety A if every member of B outside A generates B. The
variety {2x = 0,xy = 0} is the unique equationally complete variety with precisely
two immediate covers in the lattice of all associative ring varieties. The variety
of all Boolean rings is first order definable in the lattice of all associative
ring varieties. So are the varieties defined by {2x = 0,xy − yx = 0} and
{xy = 0}.