UN:外螺纹设计牙型根部可平可圆(A flat root contour is specified ,but it is necessary to provide for some threading tool crest wear, hence a rounded root contour cleared beyond the 0.25p flat width of the Basic Profile is optional.)
UNR:外螺纹设计牙型根部必须为圆弧(In order to reduce the rate of threading tool crest wear and to improve fatigue strength of a flat root thread, the Design Profile of the UNR thread has a non-reversing continuous curved root tangent to the thread flanks at a depth of 0.625 H.)
UNC:粗牙系列(Coarse-Thread Series);
UNF:细牙系列(Fine-Thread Series);
UNEF:超细牙系列(Extra-Fine-Thread Series);
UNS:特殊螺距系列(Selected Combinations和 Other Threads of Special Diameters, Pitches, and Lengths of Engagement);
另外还有恒定螺距系列(Constant Pitch Series),有8UN、12UN、16UN等等,不一一列举