Make one measurement and you have its opposite immediately. The Lid Master does the work of two calipers with far greater speed and accuracy! Plus, you can reassemble the screw in the lower set of holes and it instantly shows how much a pot will shrink during firing. This allows you to size your finished work precisely or make lids for already fired pots. With the 12 1/2% shrinkage ratio there is no need for a conversion ruler.