Thread tracing is a special tailoring technique used to accurately transfer marks to fabric. For example, if you have darts on the back pieces of your pants and you want them to be positioned exactly the same on both pieces, then you can use thread tracing. Place the two back pieces on top of each other and use thread tracing to transfer the marks (including the marks for the darts) from one layer of fabric to another. If you put on your pants later, they will fit perfectly, the darts on both sides will be positioned symmetrically.
Check out my thread tracing video tutorial, to see how it’s done. You can also double-thread your needle and sew with alternating classic stitches and stitches with loops… A special stitching thread / basting thread is usually used for thread tracing.
Learn more about stitching / basting here: How to baste fabric (simple & correct technique)
I feel that thread tracing is no longer so popular today, but I use it for practically all the time. It is the most accurate technique for transferring marks to individual pieces. It is more time consuming, but definitely much more accurate than pins. 🙂
Have a nice day, Peťa