Grade A (pitch-line) and Grade B (grouped) measuring cylinders for checking external screw threads of Unified and ISO inch, Whitworth, B.A. and ISO metric form by means of pitch diameter measuring machines. Bases for the diametral tolerances. Relative merits of grades A and B. Measuring ranges, material and finish, form and general dimensions, accuracy and identification; tables of nominal diameters and associated P values for Grade A cylinders; limits of size for Grade B cylinders.