UN Thread Types

UN (Unified) is a specification of the shape and dimensions of the thread for a given size (diameter) of fastener and number of threads per inch of fastener length. UN bolts and machine screws are the type of fastener in everyday use in the US, UK, and Canada. UN, UNR, and UNJ thread dimensions are measured in Imperial units. The M and MJ threadforms are similar but are measured in metric units and are manufactured in metric sizes.
http://www.boltplanet.com/common.htm

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Note added at 22 mins (2012-10-03 06:49:02 GMT)
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См. также:
http://en.wikipedia.org/wiki/Unified_Thread_Standard
http://www.multitran.ru/c/m.exe?t=5287104_2_1