In situ GIWAXS measurement were performed to understand the complex changes occurring during the film crystallization by thermal annealing, especially the development of the various 2D phases. Two different incident angles, namely αi = 0.3° and 2°, were employed to investigate the evolution of the upper and the whole perovskite layer, respectively. The set-up scheme and the detailed description of in situ GIWAXS experiment are provided in Supplementary Figs. S9 and S10 and Note 4. Supplementary Figure shows the integrated all frame results of the fully annealed MHP films. The eight examined systems exhibited up to four distinct phases: 3D, 2D n=1, quasi-2D (n=2), and quasi-2D (n=3). To understand the volume variations of these phases during annealing in each perovskite system, we monitored the four diffraction peaks representing the above four phases: the (001) diffraction of 3D phase, the (002) diffraction of n=1 phase, the (020) diffraction of n=2 phase, and the (020) diffraction of n=3 phase. It is worth noting that the 3D-phase (001) diffraction is sometimes affected by the superposition of diffraction intensities of other phases during tracking, such as the high-n phase or quasi-2D (111) phase in N-2/3/5/6/9/30 systems. Therefore, we hereafter use (001) diffraction dominated by 3D phase (3Dd for abbreviation) to describe it. The analysis of the eight systems leads to the unprecedented conclusion that different phases have unique growth directions (Supplementary Fig. -15 and Note 5). Specifically, the 2D/quasi-2D phases exhibit a downward growth, while the 3D phase grows laterally. This point was verified by investigating another spacer, butylammonium, for N-2 which exhibited the same behavior (Supplementary Fig. ).