Outside calipers are used to measure the external size of an object.They can provide a high degree of accuracy and repeatability. Outside calipers are especially useful when measuring over very large distances, considering if the calipers are used to measure a large diameter pipe. A vernier caliper does not have the depth capacity to straddle this large diameter while at the same time reach the outermost points of the pipes diameter.It is apparently impossible to directly measure the diameter of a shaft or other circular part using a straight rule. A tool often used in combination with the rule for this and other purposes is a pair of outside calipers.( traditionally, a single tool is termed a â€śpairâ€?of calipers, just like a â€śpairâ€?of trousers.)Outside calipers come in essentially two types: those that hold their setting by friction at the joint between the two legs and those that are spring-loaded toward the â€śbigâ€?end of their range with a screw and thumb-nut arrangement to close the calipers against the spring resistance. One variation on the friction joint type provides a thumb-nut to adjust the amount of friction and to permit locking the setting. Finally , for situations where the calipers have to be withdrawn over an obstruction that is larger than the dimension being gauged , there is a version with a thirdâ€?transferâ€?leg that can be locked in place. One of the measuring legs can be swung out of the way, and set back against the transfer leg once the tool is free.