Bobbin thread is the lower thread carried on a bobbin that already fits the machine. The best choice depends on what the lower side of the stitch should look like, the thread fiber and size the machine can handle, and whether the project benefits from matching or deliberately different top and lower threads. A finer
Topstitching thread is chosen first for how visible the finished stitch should look, then for fiber, size, finish, and whether the sewing machine supports that thread class. A thicker spool is not automatically a better topstitching choice, and a heavy-duty or bonded thread may cross into a different machine requirement entirely. Product labels and model
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