Originally Posted by gale
Here's some info I found
Embroidery Threads Cotton perle (pearl) embroidery threads, although generally softer and more shiny than crochet cotton, may be used in crochet patterns as follows:- #5 perle is a little thicker than #10 crochet cotton but is suitable for most #10 cotton patterns; #8 perle is about the same as #20 crochet cotton and #12 perle is about the same as #40 crochet cotton.
from here:
http://www.crochetaustralia.com.au/webcontent3.htm
Thank you for the information.