As seen in the Bra-makers Manual Volume 2! Use The Knob to press open bra cup curves easily and without touching any other part of the bra. We use this in our school all the time. It also makes a great surface for placing and hot-fixing crystals on any garment. The curved surface allows pressure to fix the crystals without touching the hot tip to the background fabric.
The diameter of The Knob is 2.75″