This type of caliper is used mostly in bowl and vessel turning. The wall thickness can be clearly seen at the opposite end of the caliper. This simplifies the process of obtaining a consistent wall thickness. The 7" Calipers are shaped differently to reach into various shapes of vessels.. The calipers are sized by the depth they will reach. The 4" calipers are 8" in overall length, the 7" are 14" long. The 10" measure 20" overall.
