A spanner is a type of wrench with an opening and sometimes little teeth: you can clasp it over the nut or bolt and get a good grip. In the US, the main difference between a spanner and other wrenches is the spanner is adjustable and works with many sizes of nuts and bolts. A spanner is a type of wrench with an opening and sometimes little teeth: you can clasp it over the nut or bolt and get a good grip. In the US, the main difference between a spanner and other wrenches is the spanner is adjustable and works with many sizes of nuts and bolts.What is an Adjustable Spanner? An adjustable spanner is an open-ended wrench with a moveable jaw. Its function is the same as any regular spanner - to grip fasteners, such as nuts and bolts - however, only adjustable spanners can grip fasteners of all sizes, thanks to their moveable jaw. What is an Adjustable Spanner? An adjustable spanner is an open-ended wrench with a moveable jaw. Its function is the same as any regular spanner - to grip fasteners, such as nuts and bolts - however, only adjustable spanners can grip fasteners of all sizes, thanks to their moveable jaw