UN：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：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：Coarse-Thread Series
UNF：Fine-Thread Series
UNEF：Extra-Fine-Thread Series
UNS：Selected Combinations Other Threads of Special Diameters, Pitches, and Lengths of Engagement)；