Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable deformations including strain, dislocations, or wrinkles, which can strongly affect their mechanical, optical, and electronic properties. Transmission electron microscopy (TEM) and its related techniques have become indispensable tools in uncovering the structure and subsequent physical properties in these 2D materials, atom-by-atom. Here, we utilized a combination of atomic-resolution ADF-STEM and four-dimensional (4D) STEM mapping techniques to address how different 2D materials merge to form lateral heterostructures, specifically between two distinct transition metal dichalcogenides (TMDs) at various scales (Fig. 1).