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.