Abstract: An adapter blank is provided which is adjustable in size to join generally tubular structures of different diameters, such as pipe insulation cladding, together. The adapter blank includes a tapering wall having a rim defining a larger diameter matching larger diameter pipe insulation cladding to be joined by the adapter. An overlapping edge extends away from the rim. Graduations are located at various different constant distances away from the rim. Indicia, such as numbers, are preferably provided adjacent at least some of the graduations to indicate the various different smaller diameter pipe insulation cladding sizes. The adapter blank is cut along the graduation corresponding with the smaller diameter pipe insulation cladding to be joined with the adapter. The adapter blank is thus modified into a finished adapter ready for installation joining together pipe insulation cladding, or other tubular structures of different diameters; or terminating an end of pipe, tank, vessel or other tubular structure.