Clamps onto beam 5/8" to 1-1/2" thick of any length. * General"s adjustable trammels accurately measure and mark distances that are too great for conventional compasses and dividers. * For layouts, marking templates, and drawing large circles. * 6" off-center eccentric points provide fine adjustment. * Points can be replaced with pencils, ballpoint pens, crayons, or soapstone sticks for marking various materials.