The Gilson Concrete Crack Monitors monitor crack widths in concrete buildings, bridges and roads. The overlapping upper plate is transparent and marked with a red crosshair cursor and the white bottom plate measuring grid is marked in millimeters. The HM-634 Monitor is vandal-resistant clear polycarbonate with a range of 25 mm horizontally and 10 mm vertically. Spigots on both plates enable exact measurement to .1 mm with calipers. A second Crack Monitor may be positioned for monitoring in three dimensions. The HM-635 Corner Adapter enables Crack Monitors to be used in corners with greater or less than 90 degree angles. The HM-637 Economy Crack Monitor has a range of 20 mm horizontally and 10 mm vertically, there are no spigots for use with calipers and readability is 1 mm. The HM-638 Displacement Monitor measures out-of-place movement across a crack and also measures opening and closing of the crack itself. Displacement range is 110 mm. The HM-636 Optional Crack Width Gauge assists those who survey and report on damaged buildings with an acrylic gauge scaled to measure cracks up to 7 mm and wider in corners and awkward locations.