Threads Per Inch, a specification of bolts.
http://174.143.245.138/?tag=unc
See http://www.engineershandbook.com/Tables/uncunfthreadsperinch.htm for more.
A common mistake in the US is to refer incorrectly to TPI. For instance, a number 6 bolt with thirty-two threads per inch is a "Six thirty-two bolt" not a "Six thirty seconds bolt". The correct name is written 6-32.
http://174.143.245.138/?tag=unc
See http://www.engineershandbook.com/Tables/uncunfthreadsperinch.htm for more.
A common mistake in the US is to refer incorrectly to TPI. For instance, a number 6 bolt with thirty-two threads per inch is a "Six thirty-two bolt" not a "Six thirty seconds bolt". The correct name is written 6-32.