Abstract: A device preferably in the form of a T-square having an elongated body and a perpendicularly arranged head portion and further including a plurality of staggered, offset scales extending along the length of said body from the head portion towards a distal end thereof. A plurality of scale indicators equal in number to the number of scales are provided to slide along the length of the body in aligned indicating relation with one of said plurality of scales. The device is specifically structured to locate, dimension and mark the proper locations for the formations of score lines in a multi-sided conduit or duct formed from a planar sheet of duct board wherein the device may be preset and maintained for the successive marking of a plurality of said planar duct board sheets without cause to reset the device each time a separate duct board is marked.