Unified coarse thread series UNC ¼ in to 4 in diameter; Unified fine thread series UNF ¼ in to 1½ in; Unified extra fine thread series UNEF ¼ in to 1 11/16 in. Standard constant pitch series for a graduated system of diameter/pitch combinations. Basic and design forms. Tables of basic dimensions, limits and tolerances for all diameter/pitch combinations in UNC, UNF, UNEF and standard constant pitch series given for the three classes of internal and external threads. Bases of tolerances for Unified screw threads; five sizes below ¼ in diameter for attachment purposes and for general use in aircraft appended. Evolved through international discussions between UK, Canada, USA. In accord with corresponding sections of American standard ASA B1.1-1960.