In this paper the authors present their two findings: (1) Mechanism underlying the mixed-product zone. The authors' theoretical analysis indicates that the transformation of any phase product follows its own peculiar C-curve and that, when two C-curves intersect, mixed product zone appears. In the experimentally determined TTT diagram of Cu-Zn (26.84wt.%)-Al(4.22wt.%) alloy, the C-curves of the ... [Show full abstract] bainite α1 (plate-like, 9R structure) and the equilibrium phase α (rod-like, fcc structure) intersect at the bay temperature. Fig.3 of this paper is clearly different from the corresponding TTT diagram given in literature in that the lower C-curves corresponding to α1 does not show a cutoff at the temperature (bay) of the intersecting point of the two C-curves, i.e. the transition temperature, B varies with transformation time even at a temperature higher than the bay temperature. Therefore, the existence of the mixed product zone is inevitable in the process of bainitic transformation. (2) Definition of stage of pure bainitic transformation. From the explanation just explained in (1), it naturally follows that the needed definition is: pure bainitic transformation for Cu-Zn (26.84wt%)-Al(4.22wt%) alloy occurs only when the point representing temperature and time falls within a certain area.