A hob is a fluted screw, having several narrow flutes, which are generally parallel to the length of the hob, the number of flutes in a hob two inches in diameter being usually thirteen or fifteen. A hob may be termed a steel plug, having at one end a screw in which several cutting edges are formed by means of fluted grooves, and having at the other end a square head. This head is that which is held while the hob is in use, whether by means of a spanner, or by the hob being rotated in a lathe. Hobs are, at the present time, a class of standards from which several other instruments for screwing are made, such as comb screw tools, one-point screw tools, dies, screwed plates, and taps. Sometimes a lot of hobs, consisting of about twenty, are kept as standards for reference and measurement, but no other use is permitted them, for fear of injuring or reducing them below their respective dimensions. A hob which is intended for die-making is sometimes held in a vice, the square head being gripped and the screw part extending upwards; while thus fixed, the die-making is effected by revolving the dies around the hob. The two most useful shapes for hobs are indicated by Figs. 308 and 313.