Thank you for replies. I'm from Slovenia and wasn't able to find Gütermann online in my country, I would probably have to order it from Austria or Germany... I guess I'm going to give coats duet a try then... I knew there were different thicknesses of threads but didn't know you might need different ones for different thicknesses of a certain material (ex.: cotton)... This sounds complicated. :/ I guess that's something I will learn eventually with practice.
I know on a pack of needles it says what thickness they are for and in the manual it says roughly for which material which size is for, but what about threads? Does it say on the thread what thickness it is? How do I determine if it will be too thick or too thin for a particular material? What about when you're hemming clothing and you get 3 or 4 layers of fabric, do you have to change the needle and thread or not? This is so confusing to me, I have no idea what is considered thick and what thin in terms of layering fabric.