What are 'PG' sizes?

[gez]

I was looking at a 'multicut step drill' (uni/varibit) in a on-line catalogue which cuts 'PG' sizes 7,9,11,13.5,16,21.  What the hell is PG, and is this suitable for drilling boxes?  Are these numbers the diameter in mm or something? (doesn't look like it!)  Help!

edit: page 781 of this catalogue:

http://www.rapidelectronics.co.uk/PDFs/CATPDFs/October2003/44_Tools_Power%2010.03.pdf

[Peter Snowberg]

PG sizes are for threads on steel conduit. I don't know what the letters stand for but I think it may be "pipe guage".

Take care,
-Peter