One of the numbers is from the European sizing system and one is from the American sizing system.

I guess I could ask a more specific question... I bought a spool of YLI Silk Thread for an upcoming machine quilting class. I have no idea which needle to use. The class list doesn't say which needles to use with the silk thread. Which needle would you pick?

watch out--i read here on the board that a well known brand was now being made in china and they are rubbish

The recent discussion that I seem to ecall was that the "well known" brand was now being made in India.
Further that a letter from the said company confirmed it as true.
They also continued on that the quality had not changed and the standards were just has high.

I seem to recall that the poster of the info stated that she was quite happy and would continue to buy this "well known" brand.

I read that the first set of numbers are European size (75/11) the second is American.

I just attended a machine quilting class. Our list of supplies included a size 12 'Topstich' needle. The instructor explained that the topstich needle has a larger hole/eye, and the grove is longer. This is less stressfull on the thread. She also explained that the smaller hole/eye causes the thread to fray/break. I know this because I have used size 75/11 universal needles and the hole/eye is 1/3 the size of the 'Topstich' needles and the thread frayed and broke often. During class with the size 12/Topstitch, it never broke. Enjoy your class.

I have used the Denim needles for years --for all types of sewing. Their scarf and hole are supposed to be the same size as the topstitch but they are a bit sharper. I use ballpoint for knits since they do not break the fiber. When I have a few extra $$$ I buy the titanium needles and have had good luck with them.