_Answer_. It is the distance the thread progresses lengthwise of the screw in one revolution. Thus if a single-threaded screw has one-eighth of an inch pitch, it means that the threads are ¹⁄₈ of an inch apart measured from the centre of one thread to the centre of that next to it, and therefore there are eight threads to each inch in length of the screw. QUESTION 337. _What is meant by a “single-threaded” screw?_ _Answer_. It means a screw with but one thread instead of two or more. Thus if we take a string and wind it around a pencil, it will represent a single-threaded screw, and if we take two or three strings and wind them parallel to each other, they will represent a double or treble-threaded screw. The latter kinds are seldom or never used on locomotives, so that in the following discussion only single-threaded screws will be referred to. [Illustration: Fig. 200. Full Size.] QUESTION 338. _What is the usual form of the threads of screws?_ _Answer_. The most common is what is called the V-thread, represented in fig. 200, which is made sharp at both the top and bottom. If such a thread for one screw is made very pointed and that for another is blunt, it is plain that the nut for the one will not fit the other accurately, and also that if a nut has eight threads to the inch, it will not fit on a bolt with nine. Owing to the fact that, until within a few years, no common standard has been agreed upon for the form, proportions or pitch of screws there has been very great diversity in these respects in the screws which have been used in the construction of locomotives and other machinery. In 1864 the inconvenience and confusion from this cause became so great that it attracted the attention of the Franklin Institute of Philadelphia, and a committee was appointed by that association to investigate and report on the subject. That committee recommended the adoption of the Sellers system of screw-threads and bolts, which was devised by Mr. William Sellers, of Philadelphia. This same system was subsequently adopted as the standard by both the Army and Navy Departments of the United States, and then by the Master Mechanics’ and Master Car Builders’ associations, so that it may now be regarded, and in fact is called, the United States standard, but the design is due to Mr. Sellers. QUESTION 339. _In establishing a standard system of screws and threads, what is the first thing which must be determined?_ _Answer._ The number of threads to the inch, or the pitch of the threads for screws of different diameters. [Illustration: Fig. 201. Full size.] QUESTION 340. _What is the standard for the number of threads to the inch for the different sized screws of the Sellers system?_ _Answer._ The number of threads with their other proportion is given in the table at the end of this chapter on page 348. QUESTION 341. _What is the form of the thread of this standard?_