Let ω 1 : X → X 1 , ω 2 : X 1 → X 0 be two covering maps and suppose ω = ω 2 • ω 1 is normal. Therefore, the covering map ω 2 is normal if and only if for its deck transformation groups of covering spapce X, Deck(ω 1 ) = G 1 is a normal subgroup of Deck(ω) = G. Further, by first isomorphism theorem, there is: