A curved ring spanner, or curved box-end spanner, is a spanner with an enclosed opening that grips the faces of the bolt or nut. The recess is generally a six or twelve point opening for use with nuts or bolt heads with a hexagonal shape. Eight point spanners are also made for square shaped nuts and bolt heads. Curved ring spanners are often double ended with a curved shaft to improve access to the nut or bolt. They come in a variety of different sizes and in both metric and imperial measurements.

Also see:
Ring Spanner