Tom and Wilson gave good links.

History, As I Know It To Be:
Early in 20th Century, machine tool industry and automobile companies
needing machine tools developed a new screw bolt system (along with drill
sizes with numbers and letters) for their needs. This need had to do with
the torque applied to the machine bolts, which in turn required greater
holding power on the threads before the threads failed. Bolts in the new
standard had improved thread dimensions with complex relationships between
pitch, depth, and other factors (like the steel used.) Out of that need
came the Unified Thread Standard
http://en.wikipedia.org/wiki/Unified_Thread_Standard

This is from a book for introduction to Machine Shop:
"The term 'machine screw' refers to a system of numbering used to identify
machine screws smaller than 1/4 of an inch. The number system refers to a
series of individual screw sizes within both the Unified National Coarse
(UNC, or NC) and Unified National Fine (UNF, or NF) Series..... for
example is identified by the number 12-28. The First part for he number
specified the gage size of the body of the screw. The second part of the
number specifies the number of the threads per inch. ... For Machine screw
0-80, 0 specifies the size (diameter, 0.060 inches) of the screw, and the 80
specifies the number of threads per inch.... In the number series, diameters
range from 0.060 to 0.216 inches (for the UNF standard.)"

Another article at www.wikipedia.org stated that in 1949, British, Canadian,
and USA merged tread standards and included the imperial measurements bolt
sizes into the UNC, and UNF threading standard for pitch, depth, and so
forth.

Way more than you asked, I know.

Phil

"Phil Hansen" wrote in message
news
I need some knobs etc for a few jigs.
I am in a metric country and cannot get knobs etc like your mail order
companies have and need to order.
I know what 1/4-20 and 5/16-18 means in Imp terms but what is a 10-24
thread?

Thanks