What component is placed on the shaft of the bolt before installing it in an assembly?

A washer is placed on the shaft of the bolt before installing it in an assembly to provide a smooth surface under the bolt head or nut and to help distribute the load over a larger area. This helps prevent damage to the material being fastened and reduces the risk of wear or deformation at the interface. Additionally, washers can help prevent loosening due to vibration by acting as a cushion, which increases the overall stability and integrity of the assembly. Using washers is a common best practice in aerospace assembly to ensure safety and reliability in critical components.