These were designed to do one perfect 1/4" echo but they have so many uses. This is a versatile tool and comes in many sizes. The numbers are based on a circle.... so the ProEcho 4... the curve comes from a 4" circle. You can use them for spines, continuous curves, curved crosshatcing, block designs... anywhere that you need a curve.
If you like the Divide and Design method, it is recommended that you have these!