Flat Washers or Round Washers as they are sometimes called would be by far the most commonly used washer by all trades for all types of fastenings. They are available in both metric and imperial sizes and come in a large number of nominal hole sizes and a large variety of outside diameters and thickness's available to choose from. The imperial 1-1/4" and 7/8" outside diameter washers are commonly called Penny and Half Penny washers respectively. Sometimes they can be called fender washers.