Thread-size labels are useful only after you know which numbering system they belong to. A 40 wt label, No. 40 label, and Tex 40 label do not describe thread size in the same way, even though the printed number is identical. The practical comparison is to identify the system, note fiber and ply construction, then
All-purpose thread is the sensible starting point for ordinary domestic machine sewing, but the label is not a promise that one spool fits every fabric, stitch, or machine. The useful check is simple: read the fiber and size on the spool, compare them with the seam you are making, then ask whether care, appearance, movement,
Sewing thread types make more sense when you separate four questions that spool labels often compress together: what the thread is made from, how it is constructed, how its size is expressed, and what job it is intended to do. A polyester core-spun thread, a mercerized cotton thread, a metallic composite thread, and a water-soluble