Across powders, liquids, and crystals, X-ray diffraction formed a unified framework for crystal structure determination, with Debye-Scherrer powder diffraction standardization and complementary Fourier and Patterson analyses. Electron diffraction matured as a parallel modality, yielding lattice spacings, symmetry assessments, and microstructure insights in metals and very thin films. Theoretical and general frameworks for crystals and molecular structure emerged, including vibrational spectroscopy in crystals, electronic structure of metals, and diffraction theory for liquids and solids; crystal thermodynamics and chemical crystallography advanced the design and identification of molecular crystals, illustrated by glucosamine hydrobromide structure and Patterson-method-based design. Historical Significance: The period featured pioneering works that crystallized foundational concepts: Diffraction of Electrons by a Crystal of Nickel established electron diffraction as a core crystallographic tool; The Scherrer Formula for X-Ray Particle Size Determination linked X-ray peak broadening to crystallite size, catalyzing nanoscale crystallography and quality control. The two-dimensional Crystal Statistics model with an order-disorder transition provided a theoretical basis for phase behavior in crystals, while analyses of X-ray diffraction by distorted crystal aggregates enabled defect-aware interpretation of patterns. Studies on spinel oxides highlighted how cation ordering governs structure and properties, laying groundwork for spinel crystallography and materials design.