These little template testers compensate for the diameter of the hopping foot of your longarm machine. Since the needle is a quarter of an inch away from the edge of the hopping foot, it can be hard to plan out designs using rulers and templates. With these tube template testers you can slide most any marking pen inside the tube and it will force the tip of the marking device to be one quarter inch away from the edge of the tube. This will allow you to trace around your rulers and templates to get a clear idea of exactly how they will stitch out. They are helpful for practicing on white boards or design grids, instead of on the machine and having to rip out the stitches!