There are many kinds of bolts and screws to meet different needs and in order to specify a particular grade of bolt or screw it is necessary to mention;

Fig. 117.   Measurement of Pitch of Screw.

Fig. 117. - Measurement of Pitch of Screw.

(a) Shape or form of thread.

(6) Pitch, or number of threads to the inch.

(c) Shape of head.

(d) Outline of body, barrel, or stem.

(e) Diameter.

(/) Direction of thread.

(g) Length.

(h) Material.

Before 1861 every manufacturer had a peculiar thread that he made for his own work. The result was that the large number of threads caused confusion among engineers and machinists. To avoid this it was proposed to have a standard form; today each country has a standard of its own.