Flat head pins are necessary to do stack-n-whack quilts. When doing stack-n-whack quilts, normally you have (for example) eight layers of fabric cut on the design repeat of the fabric. So you have eight exact repeats on top of each other. You take the pin and pin through one design in that repeat lining it up at that same design on each of the other seven pieces of fabric in the layer and pin. After you have done this in numerous places, you take your ruler and cut through all eight layers in a particular way. These pieces form a kaleidascope. In order for your ruler to lay flat and not roll or move, you must have flat head pins.