First there is a "semidirect" subgroup, the orthorhombic group, [2,2 ] ( or ), abstractly Z 2 × Z 2, of order 4. When the + superscript is given inside of the brackets, it means reflections generated only from the adjacent mirrors (as defined by the Coxeter diagram, ) are alternated. In general, the branch orders neighboring the + node must be even. In this case [2,2 ] and [2,2] represent two isomorphic subgroups that are geometrically distinct. The other subgroups are the pararhombic group [2,2] ( or ), also order 4, and finally the central group [2,2 ] ( or ) of order 2.