Headings on a thread pitch chart include thread class, the fit when a screw is being placed in an also-threaded part; major and minor diameter; and pitch diameter, the point where the width of thread and space between threads is equal. Screw dimensions vary based on the intended use.
Thread class is rated 1, 2 or 3 with 1 indicating easy insertion, good when something needs to be put
together or taken apart quickly, and 3 indicating a tight, highly accurate fit. Thread class may also be accompanied by either A for external threads or B for internal threads. Major and minor diameter measurements and pitch diameter include subheadings of basic, maximum and minimum. These subheadings refer to possible variance in thread size from the basic or technically precise measurement. Thread dimensions falling between maximum and minimum are required to meet size specifications.
The thread shape or form is another defining dimension when selecting screws: NC or UNC indicates a coarse thread, NF or UNF a fine thread, and UNEF an extra fine thread. Coarse thread screws are for general use, fine thread screws are used for tighter fits and extra fine when the length of thread engagement is small.