In mendellv's periodic table why they have made subgroups i.e. A & B ...?

In Mendeleev's periodic table, there are 8 groups and each group is divided into two sub-groups designated as A and B. Mendeleev arranged the elements according to their atomic weights. He found that there occurs a periodic recurrence in certain elements in terms of their physical and chemical properties. He found that elements present even in same group have different properties. So he divided the group into two subgroups containing elements having same physical and chemical properties and named them A and B