Air Gap: An air gap faucet which contains an empty space built in the base of the faucet. Air-gap faucets have three lines attached to them. (1) line from the tank to the faucet (2) a water line from the brine water side of your RO. (3) a drain line for the brine water that attaches to the drain saddle. Please check your local code to determine if an air-gap is required. Additional Details ------------------------------ Product description: Air Gap: An air gap faucet which contains an empty space built in the base of the faucet. Air-gap faucets have three lines attached to them. (1) line from the tank to the faucet (2) a water line from the brine water side of your RO. (3) a drain line for the brine water that attaches to the drain saddle. Please check your local code to determine if an air-gap is required.