A spanner is a tool for gripping or spanning nuts and bolt-heads for the purpose of screwing and unscrewing them. Spanners are used also for rotating plugs of water-cocks, steam-cocks, and valve-spindles, also for twisting metals, and holding tools while boring with a lathe. The simplest sort of spanner has but one gap, which is formed in cases of emergency by heating one end of a straight bar of iron and bending it to make a gap of the width required.

A simple class of spanners in general use is represented by Fig. 341, having but one gap end attached to a straight lever named the handle. With the object of making one spanner suitable for nuts or bolt-heads of two sizes, the tool is provided with two gaps, one at each end; such a spanner is indicated by Fig. 342. The most convenient spanners for use are those which are thick at the gap parts, but not broad; most of the gap spanners in use being too thin and too broad, which prevents them being used in corners where there is not much room. The gap-sides of spanners are smoothly polished and made to fit the nuts with only sufficient room to allow a spanner to be quickly put to a nut and taken away from it during use. When it is needful to use a spanner for nuts that are too small for the spanner's gap, the mode of making them fit each other consists in placing a packing plate or filler between the side of the nut and the gap-side; and, if necessary, two or three such pieces may be used for one nut, by which means a spanner whose gap is half an inch too large may be used with a filler which is half an inch thick. A number of these pieces should be made, each about two and a half inches long, and of various thicknesses, so that they may be kept ready for use. To prevent injury to polished iron nuts, and to gun-metal nuts, it is necessary to make the fillers of sheet brass, and to polish them.

When it is requisite to use a spanner for a nut or head which is below a floor, or a foot plate, or beyond a wall, a socket spanner is used. Such a spanner may have a short bent handle for small nuts or heads that are only a short distance from the operator, and the tool is indicated by Fig. 343. Fig. 344 denotes a socket spanner having a straight handle with a hole in its end to hold a lever which is put into the hole when the spanner is on the head ready to be rotated; such a tool is therefore convenient when the object to be fastened or unfastened is in a corner or near a wall. Fig. 347 denotes a tee-handle spanner, the entire tool being in one piece, and used in places where there is sufficient room for the handle to rotate. The claw spanner shown by Fig. 348 is employed for nuts or heads that are situated in corners, gaps, slots, and similar places. The spanners represented by Figs. 345 and 346 are a class of strong spanners with only a small thickness of metal, but these can only be used where there is sufficient room for the spanner to entirely encircle the nut.

A very useful class of spanners is that named screw spanners, which may be made to fit nuts and heads of several different diameters; these tools are represented by Figs. 349 and 3.50. The gap of the variable spanner shown by Fig. 349 is made to open or close to the desired width by rotating the screw shown by S with a thumb and finger; consequently, this tool is not so easily handled as the one shown by Fig. 350, which is adapted to nuts of various sizes by rotating the handle, and this may be done while the spanner is around a nut, so that both hands of the operator may be used for rotating the spanner-handle.