Doryman Shellfish Caliper &ndash; Floating Ruler &ndash; 3&ndash;6.5 Inch (Blue) Measure the legal fishing sizes of lobsters and crabs, 3 to 6.5 inches Extends to a 9 inch long straight edge rule, collapses to fit in a pocket Made of expanded PVC, which allows the tool to float Handy lanyard provided to help eliminate accidental drops Easy twist knob can lock a measurement in place