There are two paths to apply thread to work. The most used on a sewing machine is top thread – it follows the usual thread path from the spool to the needle, and through the magic of the lock stitch is looped around the bobbin thread and drawn back to the top of the work.
This little video shows clearly all the moving parts that make up a stitch. You can see that the top thread travels through several steps to get to the fabric. It also gets pulled back and forth through the eye of the needle as the take-up releases it for each stitch and takes it up again. This wear can be absorbed by finer threads running through appropriate sized eyes, and a wide range of threads can be used.
In a perfectly counter-intuitive move matched only by the wire industry, thread weights are inverse to the actual size of the thing in question. The 50wt thread on the left, from Superior, is the finest I tend to use because anything smaller than that does not cover the underlying fabric in any useful fashion. The weights increase to 40wt, also from Superior, 30wt, from Sulky, and 12wt from Sulky. All these are 100% cotton threads, and three of the four are also variegated threads.
Once threads are too coarse to fit through the eye of a needle, they can be wound on a bobbin and worked from the back of the piece. I’ll take some pictures of that process and demonstrate in the next bench notes.