Uncovering the origins of the transition behaviors requires understanding the molecular and structural modifications during the phase transitions. To that end, we performed in situ grazing incidence X-ray diffraction (GIXD) during thermally triggered polymorph transitions. A GIXD video of the I-II transition was obtained by continuous heating at 5 °C min -1 and capturing the diffraction pattern every 5 seconds (Movie S7). The unit cells for polymorphs I and II were extracted from the GIXD patterns via a regression fitting the diffraction pattern as discussed in previous work . This fitting for polymorph I also matched the single crystal unit cell obtained at room temperature . During the I-II transition, we observed minor changes to the unit cell, showing peak movements in the q z direction while keeping the Bragg rods in place and maintaining the monoclinic symmetry (Figure ). The largest extent of peak shift in the q z position occurred right before the I-II polymorph transition, similar to pretransition behavior observed by Panda and Naumov et. al. (Figure ). This pretransition behavior is typical of pretransition regions of critical phenomena , such as in the lattice fluctuations necessary to drive the avalanche behavior . Comparing the fitted unit cells for polymorphs I and II (Figure ), there is a significant increase in the β-angle during the I-II transition, consistent with the observed out of plane shift. By comparing single crystal X-ray diffraction with GIXD for polymorph I, we showed this angle reflects the molecular tilt with respect to the substrate, measured to be 66° (Figure ), commensurate with the complement of the β-angle, measured to be 67°3 . During the I-II transition, the β-angle changed from 113° to nearly 90°, suggesting the molecules standing straight up from a more tilted position, like reverse dominos.