These are the gauges that Dr. Frost uses place brackets consistently for smile arc! The set comes with 3 gauges, each with different measurements on each end (4mm, 4.5mm, 5mm, 5.5mm, 6mm, 6.5mm). The thin piece of the gauge end is designed to be placed in the center of the bracket slot and the numbers on each side indicate how many millimeters the bracket will be from the incisal edge of the tooth to the center of the slot. If you have questions, email email@example.com.