The Starrett PT22431 depth attachment stabilizes the caliper while measuring the depth of grooves and bores. This attachment allows for perpendicular entry of the depth rod into a hole or groove, for accurate depth measurements. It attaches to the caliper by tightening a screw and removes easily when finished. Place the caliper vertically on a flat surface when tightening the screw to ensure the end of caliper and attachment are evenly lined up, for accuracy during measurement. Attachment is compatible with Starrett 6”, 9”, and 150 mm calipers.