A couple of weeks ago I discussed sizing of needles. Today I want to talk about sizing of thread.
The general rule is: The LARGER the number, the SMALLER (finer) the thread.
And just to be clear in this discussion, I am talking about the types of cotton threads you would find in a general quilting or sewing store. Large cones of thread used for commercial sewing and general all-purpose threads found in discount stores may or may not adhere to this sizing formula (and most of them contain a polyester component).
Next to it is Mettler 50wt thread. This is the weight most used for general sewing and piecing. We carry Gutermann, Mettler, Star, and Aurifil thread in this size.
To the right of that is Mettler 60wt thread. It is slightly finer than the 50wt. I have seen it used for piecing, machine quilting, applique, and hand piecing. I personally use it for applique and English Paper Piecing, since its finer size makes for smaller stitches in handwork.
Finally, there's YLI Silk 100wt. This is the smallest thread we carry, and it is used primarily for hand applique.
Floss/Perle Cotton is sized the same way. The larger the number, the smaller the thread. Size 5 perle cotton is larger than size 8 or size 12.
One more thing. I did not include hand quilting thread in this discussion. It is in a class all its own, and is generally all about the same size. If a spool is marked "Quilting" or "Hand Quilting", it will usually have a coating on it to make it smoother to pull through all the layers of the quilt sandwich. NEVER use this thread in your sewing machine! The coating may rub off inside the machine and gunk up the mechanics.