Sice 8 - 9 - 10 - 11 - 13 - 15 - 17 - 19 mm
The open end of a spanner fits with very little play over the parallel and opposite sides of a hex or square nut or bolt head.
An open end spanner is made for a specific size of nut or bolt and the chance of damaging the bolt or nut is small because the lever length is adjusted to the size of bolt or nut.
A ring spanner uses all the corners of a nut or bolt head.
The annular jaw encloses a nut or bolt head over the entire circumference, thereby engaging all six corners. As a result, more force can often be put than with a spanner, which only grips at two corners.
The advantage of this is that it is possible to turn a nut or a bolt head in a narrow space with only a small turning stroke.