A flat form B washer is a type of washer that has a specific shape and dimensions defined by the American Society of Mechanical Engineers (ASME) B18.21.1 standard. It has a relatively flat profile with slightly rounded edges and is commonly used in applications where a reliable seal or spacer is needed between two mating parts. Flat form B washers are often used in bolts, nuts, and other fasteners to prevent leakage or wear.