Spanner

A basic spanner usually has, either, an open end and a closed (ring) end, or two of the same.
When the spanner has an open and a closed end, they will usually by the same size. The closed end gives the best grip on your nut, but sometimes, it's not possible to get a closed spanner onto the nut (for example, if the nut is at the end of a cable or pipe). In that case, the open end allows you to place the spanner onto the nut from the side and then remove it after turning.

Spanners that have two rings or two open ends usually have two different sizes, such as 9 & 10mm.