Throughout the entire book it has been the aim to eliminate all obsolete practices and methods; and to confine the treatment exclusively to the design, construction, use and adaptation of the numerous tools and systems illustrated. It has further been the aim to make these descriptions as brief and concise as possible, and always of the most up-to-date and approved methods of con struction, but never to the neglect of the fundamental principles involved. While a number of the tools taken and methods described are original, the majority have been selected from the columns of the American Machinist and Machinery. The author, therefore, begs to extend his thanks to the publishers of these journals and also to the writers of the articles collectively, for the valuable data and information which have made this compilation possible. It is hoped that the work will assist the present-day mechanic in the design, construction, and use of expedient and economical gages and gaging systems, and that it will also be the means of increasing the output and efficiency of duplicate machinery, and at the same time be instrumental in lowering the cost of produc tion on interchangeable work.