These adapters are KF-16 to 3/4 inch ID for PVC flex hose tubing. The flange size follows ISO-KF standards and is NW-16. They are commonly used to make connections between ISO-KF fittings and PVC Tubing, requires cementing. The ISO-KF flanges are often called quick flange fittings because they are quick to assemble. A circumferential clamp and centering ring form the vacuum seal between the connecting flanges (please see top right figure - click to enlarge). The centering ring contains a rubber elastomer o-ring. The standard sizes for quick flange fittings are KF-10, KF-16, KF-25, KF-40, and KF-50 with flange sizes NW-10, NW-16, NW-25, NW-40, and NW-50, respectively. They are made of Schedule 80 Polyvinyl Chloride. This is ONE KF16 to 3/4 inch ID PVC Tubing Hose Adapter Only, Hose/Tubing Not Included.