This Eisco Labs pack of 10 blue neoprene stoppers features two 5mm-diameter holes. Each stopper measures 13mm bottom diameter, 16mm top diameter, and 24mm in length. Neoprene rubber provides excellent resistance to petroleum, oils, and most inorganic acids and bases. The two-hole design accommodates glass tubing, thermometers, or other instruments for experiments requiring sealed, multi-port access through the stopper. Suitable for chemistry classrooms and professional laboratories.