T1 transitions contribute to systematic interface rotation. For each model panel, we show an associated reference real-cell image from the same image data as Video 4. (A) Four cells (1, 2, 3, and 4) in the initial state. The vertical interface is rich in myosin and as such begins to contract. (B) The same four cells, now at the stage of the higher-order vertex. As the initial vertical interface contracts, it pulls along with it the transverse interfaces around it, causing them to become slightly more vertical in their orientation. As this occurs, myosin is recruited to the formerly transverse interface. (C) The newly formed horizontal interface elongates, which further rectifies the transverse interfaces of interest. (D) Cells 1, 2, 3, and 4 have now fully intercalated and have formed new cell-cell contacts (i.e., between cells 1 and 4, which originally had no contact with each other). The previously transverse interfaces of interest have now rotated to become entirely vertical, and as such represent newly formed vertical interfaces ready to undergo an additional round of intercalation.