Wikipedia
Diameter tape
A diameter tape (D-tape) is a measuring tape used to estimate the diameter of a cylinder object, typically the stem of a tree or pipe. A diameter tape has either metric or imperial measurements reduced by the value of pi. This means the tape measures the diameter of the object. It is assumed that the cylinder object is a perfect circle. The diameter tape provides an approximation of diameter; most commonly used in dendrometry.
Diameter tapes are usually made of cloth or metal, and on one side of the tape have diameter measurements and on the other standard measurements (not reduced by pi).