This study investigates the atomic structure and properties of grain boundaries in single-layer graphene. The researchers used a combination of old and new transmission electron microscopy (TEM) techniques to image grain boundaries at the atomic scale, revealing that different grains connect primarily through pentagon-heptagon pairs. They also used diffraction-filtered imaging to rapidly map the locations, orientations, and shapes of hundreds of grains and boundaries, identifying an intricate patchwork of grains connected by tilt boundaries. The results show that grain boundaries significantly weaken the mechanical strength of graphene membranes but do not measurably alter their electrical properties. These findings highlight the importance of grain boundaries in determining the mechanical and electrical behavior of graphene. The study also demonstrates that grain boundaries are visible in scanning electron microscopy and atomic force microscopy (AFM) due to surface contamination. Using AFM, the researchers found that the failure strength of polycrystalline CVD graphene membranes is about 100 nN, an order of magnitude lower than that of single-crystal exfoliated graphene. Additionally, AC-EFM measurements showed that the electrical properties of the graphene membranes are comparable to previous results, with room-temperature mobilities of 800-4,000 cm²/V·s. The grain boundaries were found to have low resistance, with an upper bound on the average grain boundary resistance of less than 60 Ω·μm/L. The study provides a new method for characterizing graphene grains and grain boundaries on all relevant length scales, which is crucial for understanding the microscopic and macroscopic impact of grain structure on graphene membranes. The results represent a critical step forward in realizing the potential of atomic membranes in electronic, mechanical, and energy-harvesting devices. The techniques used in this study open the door to systematic exploration of the effects of grain structure on the physical, chemical, optical, and electronic properties of graphene membranes.This study investigates the atomic structure and properties of grain boundaries in single-layer graphene. The researchers used a combination of old and new transmission electron microscopy (TEM) techniques to image grain boundaries at the atomic scale, revealing that different grains connect primarily through pentagon-heptagon pairs. They also used diffraction-filtered imaging to rapidly map the locations, orientations, and shapes of hundreds of grains and boundaries, identifying an intricate patchwork of grains connected by tilt boundaries. The results show that grain boundaries significantly weaken the mechanical strength of graphene membranes but do not measurably alter their electrical properties. These findings highlight the importance of grain boundaries in determining the mechanical and electrical behavior of graphene. The study also demonstrates that grain boundaries are visible in scanning electron microscopy and atomic force microscopy (AFM) due to surface contamination. Using AFM, the researchers found that the failure strength of polycrystalline CVD graphene membranes is about 100 nN, an order of magnitude lower than that of single-crystal exfoliated graphene. Additionally, AC-EFM measurements showed that the electrical properties of the graphene membranes are comparable to previous results, with room-temperature mobilities of 800-4,000 cm²/V·s. The grain boundaries were found to have low resistance, with an upper bound on the average grain boundary resistance of less than 60 Ω·μm/L. The study provides a new method for characterizing graphene grains and grain boundaries on all relevant length scales, which is crucial for understanding the microscopic and macroscopic impact of grain structure on graphene membranes. The results represent a critical step forward in realizing the potential of atomic membranes in electronic, mechanical, and energy-harvesting devices. The techniques used in this study open the door to systematic exploration of the effects of grain structure on the physical, chemical, optical, and electronic properties of graphene membranes.