The Unified Thread Series was agreed on by the United States, Great Britain and Canada in 1949 to obtain interchangeability. Recently, modifications have been made to the original 1949 specifications for ease of use.
The Unified Thread Series is classified into coarse threads, fine threads and extra fine threads much like Metric screw threads are.
UNC: Is the symbol for Unified coarse pitch threads.
UNF: Is the symbol for Unified fine pitch threads.