Both English and Metric dial calipers use a thumb-operated roll knob for fine adjustment. when you use a dial caliper, always move the measuring jaws backward and forward to center the jaws on the jobject being measured. Make sure the caliper jaws lay flat on or aroound the object . If the jaws are tilted in any way, you will not obtain an accurate measurement. Although dial calipers are precision measuring instruments , they are only accurate to plus or minus two-thousandths (┬▒0.002) of an inch.