For an r = 0, 1, · · · , ∞, by D r (R), D r + (R), D r c (R) we denote respectively the groups of C r diffeomorphisms, orientation-preserving C r diffeomorphisms, and compactly supported C r diffeomorphisms of the real line. We think of these groups as bitopologies spaces endowed with the compactopen C r topology and the Whitney C r topology. We prove that all the triples (D r (R), D r + (R), D r c (R)), 0 ≤ r ≤ ∞, are pairwise bitopologically equivalent, which allows to apply known results on the topological structure of homeomorphism groups of the real line to recognizing the topological structure of the diffeomorphisms groups of R.2000 Mathematics Subject Classification. 57S05, 58D05, 58D15. Key words and phrases. The diffeomorphism group, the Whitney topology, the box product.